Which sets are you rebuilding? There could be any reason, you dropped it, you were a kid at some point (I know what you did), you broke it then broke down in tears, your sister stepped on your Jabba's Palace (I'm not projecting at all ) or just a nostalgia trip.
I have remade my Nuvohk in full black, and then adjusted it to look radioactive.
I rebuilt the Viking ship I have 'cause it's cool. I plan on rebuilding the MX-81 Hypersonic Operations Spacecraft because I found the wing piece I had lost so many years ago.
I tried building Tahu Mata, but I don't have the majority of the parts.
Fero & Skirminax, Gresh, skrall, core hunter, breakout rocka, some of my partly-destroyed helm's deep, Raw Jaw, and my partially-destroyed eradicator mech. That's all I can remember.
All of my Power miners sets (stone crusher, Mine Mech, Claw digger, thunder driller, and that one with the rakshi blade who's name I cannot remember)
My Lego agents sets (Sawfist's vehicle, Chase's speed boat and breakjaw's light tower, the entire swamp raid, the lava base)
The trash compactor from Toy Story
A Lego city car
A small Lego ninjago set
the Ramses Pyramid set
And I would rebuild my older Bionicle sets, but Tahu mistika and Vastus' broken joints tell me otherwise.
Me and a couple of friends found a mangled 2007 Batcave set in my school's storage room so we're rebuilding it to its former glory
In my own collection, I have rebuilt (but with some parts missing) Kopaka Mata, Lehvak, Onua Nuva, Ehrye, Nuju Metru, a Kikanalo, Hordika Onewa, Hahli Inika, Matoro Inika, Pridak, Pohatu Phantoka, Ehlek, Malum, Stronius, Ben 10 Humungousaur, and used whatever else left I could salvage to rebuild the combo/alternate models
A few days ago I tried to rebuild Onua Mistika and Mantax, but a lot of pieces were either missing or broken.
I've been rebuilding most of my collection as best as I can.
Lots of starfighters.
I don't rebuild shstem sets unless I really liked them, but I did start rebuilding all My Bionicle sets I could. So far I have a few hordika built up, Brutaka, some Inika, some phantoka, the Glatorian, and a few Metru.
I have rebuilt most of my sets in order to seal them away.
As of recent, I've rebuilt Zesk, Tarduk and Legends Mata Nui
Tahu 2015, for no reason other than to compare with 2016 Tahu.
I then destroyed him to form Quake beast.
Rebuilt all my old bonkles
I don't take apart sets that often, and if I do, they're back together within a few days. One day many years ago, however, several of my shelves collapsed, and I had to rebuild many Star Wars and Mars Mission sets. Thankfully, my UCS AT-ST was spared from the disaster.
I have 4 of my G1 Bionicle sets rebuilt at the moment: Kurahk, Pridak, Vakama Metru, and Vamprah (in that order).
In fact, I rebuilt Vamprah the day of me typing this. Finally decided to rebuild him after 8 years of having him in peices. I wanted to rebuild him because A) I have all the parts and B) Vamprah has always been my favorite Makuta. I really like his overall design.
Chirox: Him over there? That's not a makuta, that's an actual bat we decided to recruit...
Vamprah: (hisses and flies off)
I'm considering rebuilding Matau Metru.
Krika. Many times. Along with pretty much every other Bionicle set I own, though the Glatorian are a particular favorite; I still have the original canisters and/or boxes most of the sets came in, and store them accordingly. The system sets, on the other hand... well, let's just say I haven't attempted to sort out that mess in over five years.