I've read that sometimes the venue's Day-Of Coordinator mainly looks out for venue related things (they are providing catering so I imagine that would go into it). This is great, but I am also looking to have someone to make sure we are sticking to our timeline that day, to review the decorations I provide and make sure it's getting put up in the right places, and to put out any fires in general and be the contact with the vendors. I asked a friend initially to be my "point person" so she could be my eyes and ears, but she is very busy and declined.
I have a suspicion that the Day of Coordinator will do these things, but I am not sure.
I'm an event planner myself (for professional conferences though) so I also think that I have somewhat higher standards and am more familiar with the job than most people. My fiancee thinks that it's unnecessary to hire someone else because he knows how thorough I'm being with the planning, but I just want someone there on my day who I can trust will look out for me so I don't have to be worrying about it. This isn't to say that I expect "perfection" - it is inevitable that no matter how much prep you do, something will go wrong at every single event - I just would like to be able to know that someone I trust has my back.
That said, I have looked at the average cost of a Day-Of Coordinator and it would really have to provide a service I'm not already getting with my venue for it to be worth it.
We had to postpone our wedding from early September 2020 to September 2021 (in Maryland) - we're still doing our cake tasting in a few weeks though, so I'm planning on asking them more details about what their coordinator will or won't do.