I've seen them be useful when used in combination with a bug tracker. That way is investigating the same problem they are likely to find out that it has already

I've seen them be useful when used in combination with a bug tracker. That way is investigating the same problem they are likely to find out that it has already been reported. And having a todo mark a specific region of code can save time when we jump in to fix it. I have also had situations where we decided "we are releasing in a month, cut corners and we'll deal with it later", and putting in TODOs greatly help deal with it later