Thermal paste is crucial for the proper operation of any modern computer. After a few years, the cheap crud they apply at the factory often dries up or becomes otherwise more harmful than beneficial to keeping your CPU or GPU cool. Sometimes you can see this with a software temperature monitor, other times it won’t seem to run all that much hotter, but simple lock up, reboot (kernel panic, GPU panic), or present graphics corruption.
Mine was “running fine” but would simply reboot with a GPU panic whenever attempting to do something that heated it up. Once hot it was usually fine for a while. Lately it had gotten bad enough that it would reboot in the middle of the night, sleeping and cold. I had no choice but to open it up, remove several screws and a few parts, and redo the paste. ifixit.com has great step by step tutorials with photos and comments from people who have done it. This page is mostly just here to document my photos. This may be my first legit post trying to link several photos on this site.