In ME2, the reason Shepard builds models is because Kelly recommended a hobby to try and help them relax after coming back from the dead. An outlet to relieve tension between missions.

Shw actually suggested things like knitting or yoga, but Shepard shot each down quickly till she finally suggested scale model kits as an option, and EDI chimed in that Shepard would likely enjoy the process given their military background and the amount of kits available on the market.

I went to the Kenneth Griffith Museum of Science And Industry and annoyed my travel companion by staying in the boat exhibit for way longer than necessary despite it being a massive building and that was only one small part of it.

Finally built Ace's and Law's ships. I need to rebuild my big Merry.