Most of the dirt in a pool starts on the surface. Sunscreen, leaves and dust sit there until a skimmer pulls them into the basket. If the skimmer is in the wrong spot, or the weir is stuck, that dirt ends up on the floor.
On a new fibreglass install we set the skimmer to the prevailing wind and the shape of the shell. One well-placed skimmer beats two that fight each other. Keep the water at the right level so the weir can flap. Empty the basket. If you have heavy trees, a leaf net or robotic cleaner still helps.
Upgrading an older skimmer is sometimes a lid, a basket and a check of the plumbing, not a full rebuild. If you are planning a new pool, this is one of those details we handle in the set-out so you are not chasing leaves every afternoon.