Obvious "this is what works for me and why I do it" disclaimer, but:

• I have a growing collection of games which spans multiple consoles and physical/digital media alike, so I need a way to track what games I have and where, so as to not buy them multiple times (I don't believe this to be a typical use case, but I could be wrong?).
• Aside from that practical element, I have a few lists on HLTB where I track what I (might) want to play next, but realistically it's pretty much always a case of checking out what I have and picking one based on my mood / gut feeling / whatever.
• Lastly, it's about actually finishing games - getting started with some form of tracking helped me tremendously get past the "pick up and drop after a few hours" mentality (?). I wouldn't have gotten to experience some of the best games I've played in recent memories, were it not for my backlog, because I probably would have forgotten I had them / wouldn't have stuck with them.