SPOILER WARNING:

My memory for this is that the doors are there to circumvent the pits, but they are not accessible at first. As in, you have to either fall down the pits, explore down there, and then climb back up, or you have to float over the pits. Once you get past the pits, there is a lever or switch or something that will unseal the doors so you can bypass the pits in the future.

I played this a while ago though, so if anyone spots something I'm misremembering, please post to point it out.

Oh by the way, I think that the way the float spell works is that you will still fall down the pit, but you won't take damage. There's another spell that will let you float UP out of a pit also, but I'm not sure if you will have found it yet. So it might not be possible to pass the pits without falling in. Anyway, I definitely went down the pits the first time I did this, and then later found a way to open those doors.