I use it on occasion when I've got something I do believe should be on the site, but I'm on the fence or feel I may be biased, so I want a second opinion. It doesn't mean I wouldn't approve it -- I would, if someone else uploaded it and if I had approval powers -- it means I want to make sure I'm not the only one who would approve it.
For example:
- Sometimes I come across some art that really appeals to me, but I can't put my finger on why it does, and has some kind of a rough quality or flaws that would prevent me from uploading it otherwise. If it has a strong good point that I just can't explain, it will appeal to an approver too and make it through the queue. If it's just me personally seeing something that isn't there, it won't be approved. Either way, the queue served its purpose.
- Maybe I've been scouring the web for art of an unpopular copyright or character with little to no art uploaded here. I find one that seems pretty good, but isn't at the level of quality that I'd immediately go "Wow! this belongs on Danbooru" if I came across it without looking for it. I think it's worth uploading, but is it really good enough, or did I get excited by a mediocre piece after wading through a bunch of downright bad ones? Am I desperate to find something worth uploading and subconsciously lowered my standards along the way? Time for a second opinion.
- In general, sending a post I'm less confident about through the queue once in a while gives me feedback that my judgment remains consistent with the site's consensus. There's a large set of pictures that wouldn't get approved but wouldn't get flagged either, so testing the queue sometimes can help me make sure I'm not uploading things in that area.