One thing I love about #pico8 is that nearly every game automatically includes source code. I'll wonder how they achieved some effect, and can just learn from the code...
Less obvious benefit: I sometimes make mods to a game (especially the UI) to make it more comfortable for me to play-- since I often play on a phone or retro handheld, I might swap in larger sprites that are easier for my retro eyes... e.g. this quick sprite + 70 LOC mod I made to KiloWar, @2bitchuck 's remake of Mille Bornes: