I also use a tiered strategy and FIXME for things that really need to be fixed or I have a solid idea what the next step would be to fixing it.
TODOs are something less solid than a FIXME and are also just about getting it out of your head so you can devote more mental energy to something else or just relax.
Maybe the idea is not fully formed yet, maybe you are not sure you really want to do it, maybe it is waiting on something else, but it needs to be captured or you will have to keep thinking about it lest you forget.
As soon as I write down a TODO (code or other) that was only in my head, I can relax a little more, because now it's captured for future reference.
TODOs are something less solid than a FIXME and are also just about getting it out of your head so you can devote more mental energy to something else or just relax.
Maybe the idea is not fully formed yet, maybe you are not sure you really want to do it, maybe it is waiting on something else, but it needs to be captured or you will have to keep thinking about it lest you forget.
As soon as I write down a TODO (code or other) that was only in my head, I can relax a little more, because now it's captured for future reference.