In my mind, a finished book is like an ambassador that you send out into the world to entice people to your cause (that is, you’re trying to find readers). An unpublished manuscript isn’t going to do that for you. It’s not bringing any readers into the fold, accumulating any sales, building your brand, etc. All it does is gather dust, when it could be working for you.
I know that there are those who advocate a strategy of holding back until perhaps you finish two or three books and then publishing them – maybe all at once, maybe several weeks apart. I certainly understand the logic there, especially if the algorithms on platforms like Amazon end up giving you a boost because of the shortened length of time between publication dates. (Or if you’re writing serials, where each episode ends in a cliffhanger. That may be an occasion when you wait to publish, so that you’ll be able to satisfy the reader’s curiosity as to what happens next.)
However, most of us are going to require time to become successful. Time to find a niche, time to build a fan base, time to become established in our genres. Every second your book isn’t out there is lost time – and possibly lost opportunity. I think you need to give readers as much time as possible to find you. Sure, you books can sit out there forever waiting to be discovered, but – all other things being equal – forever and a day just gives you that much more of an edge.