As already mentioned people keep them closed for either the resell value or just keep em for collection, I mean unoponed sets are very valuable as years go by.
Anyway IIRC I have one these unopened, mainly cause I already had one and got this one for free. 
I still have an extra Winter Village Market that I’m probably going to sell since I have no use for that. I can never leave a good set unopened.
I feel not opening a set takes away the experience of building it for the first time. and I find it a waste of parts.
I had ordered an extra G2 Onua Master for parts and decided to grab a trans-orange Mask of Fire from that special promotion. Well, mine got lost in the mail, then found again, but by the time it was found Lego had already sent me a new Onua and a new polybag.
Of course, I opened both Onuas and one of the polybags, and I’m crossing my fingers that the one I didn’t open will be worth something at some point. But with how poorly G2 did, I might be waiting a while.
The ones I never opened are the ones my nephew did…which is Onua Master.
I have an unopened Shadow Trooper Battle Pack that I’m keeping for resale, and a Skull Basher that I’m debating using for parts.
To all those who question the keeping of Lego sets intact, I give you this:
http://www.telegraph.co.uk/investing/■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■-than-shares-and-gold/
Unopened LEGO?
What’s that? 
(not that I question the validity of such an endeavor, I simply…have chosen not to take that path myself)
Just went and looked in the closet and found that I actually have two multiples of sets that I’ve never opened,
I think the Agents one might have significant value…
I open all of my lego sets. Idk, I just feel like they don’t deserve to sit in a box, it’s a lego set, it’s begging to be built, played with, and proudly displayed.
I once got two Ikir’s. But I opened both of them. Now I realize I could have kept it sealed and sell it for s decent amount…
Only recently I realized how profitable old LEGO kept in box could be… If I will get any new sets that I don’t really need, I shall just keep them in a wardrobe from now on…
The market works differently from that now, but you can make a decent profit on licensed sets.
That’s the thing, is that lego investing has taken a huge spike in the last decade or so. You won’t see the same numbers that those sets are showing.
It obviously depends on the set and several different factors, but the game is changing.
I have always opened and built new sets right away.
I know. I want to open myslef a business where I just buy the newest licenced sets that are likely to be wanted by collectors in the future (such as that James Bond Aston Martin set), keep it in a safe place in good condition, and re-sell it after it gets retired. I can bet the Aston Martin will have a huge value in like 5 years from now…
Most of the sets in my sealed set collection are actually just waiting to be built, when I have the time and space for it 
But there’s a few I have been weary on opening:
I didn’t find any of the Walmart sets in store, but I was lucky to find this shortly after on eBay around $20-$30 before the price spiked. I have seen reviews of it opened up, and frankly I’m disappointed that there’s no special promotional material inside about the history of LEGO (even though it’s one of several sets celebrating the 60th anniversary of Automatic Binding Bricks). That said, it’s still a cool box and I may want to keep it around sealed in case I need to resell it. I can build the main model from my own parts.
After receiving this from a friend who knew I liked collecting “bizarre LEGO”, I’m just not ready to open this one up yet. When the time is right, I’ll be happy to photograph the experience. All the while and after, I’ll keep it in good shape.
Continuing the topic of “bizarre LEGO”, not only was this a harder to find clock set for me (thanks BrickLink for the assist), but also a very special dolphin. This transparent blue dolphin, released only here and in 3117 Flashy Pool (see below) had a manufacturing defect:
“Dolphins in trans-light blue were produced with a hole on the bottom that is too narrow to properly accept the two studs that would normally fit for connection. There is an internal structure in the hole that is too small for Technic axles, but which will loosely accept a bar.” [BrickLink listing of Item 6228c]
With that in mind, I’m nervous to open this one hastily for fear of breaking part of the dolphin, as well as smudging it. Again, when I do plan to open this, I will get photography of the experience as it’s fascinating that it never had a repaired edition in this color.
About a month ago I opened my sealed Nick Bluetooth and Gorm sets that I’d had for a couple year, because for Christmas I got an Allegra, and figured it was time
Back in 2018, I bought the entire Sons of Garmadon wave of Ninjago, and I only opened the final set about a month ago.
UPDATE: March 23, 2020 I opened the clock set from my previous post.
I have found a few other sets unopened, but mostly because I haven’t built them yet.
There are two sets I’d like to have sealed for sentimental reasons:
4790 Alpha Team Robot Diver. This LEGO set was featured on my first LEGO video on YouTube, and luckily I did find this one in September 2019 at a fair price.
6242 Soldier’s Fort from Pirates - aka, my favorite LEGO set of all time. I don’t have a sealed copy yet, but I still enjoy rebuilding my childhood copy. Many sets impress me and in some cases, better than this, but nostalgia aside this always stood out to me as a fun, modular playset with a great number of figures, accessories, play features and a boat and crocodile to stand on its own two legs. Even if one is a pegleg. Plus it offers a good platform for expansion with other sets in its line, like adding in 8396 Soldier’s Arsenal near the flagpole for a suitable armory.
This is the only sealed set in my collection. Bought it at the thrift store for 9.50 and didn’t directly build it. I went to a flea market the next day, and guess what I found? A built copy of the set for free.
I’m keeping this around for if I ever need the parts. The plane is nifty, but not exciting enough to have twice (nor is its B-model), but it does have nice parts. If I ever need large white panels or some yellow beams, I know exactly where to look!
