Laitance is not bleed water
The two are part of the same process, so it is easy to mix them up. Bleed water is the water itself, working its way up to the surface of a fresh pour in the first hour or so while the heavier material settles below it.
Laitance is what is left behind. As that water finally evaporates or gets worked away, it carries the finest cement particles up with it, and those fines settle on the surface as a thin, weak film that never bonded properly with the aggregate underneath. Bleed water is a stage the concrete passes through. Laitance is a residue it leaves.
Why it has to come off before anything goes over it
That top film is weaker than the concrete underneath it, and it does not stick well to whatever gets put on top of it either. A finish troweled too early works laitance back into the surface instead of letting it go, and that weak layer becomes part of what people see and walk on. A coating or a topping poured over laitance that never got dealt with is really bonding to that thin, weak film rather than to the slab itself, and it can peel or delaminate later even though the concrete underneath is sound.
Timing the finish work so laitance gets worked off the surface, rather than troweled back into it, is part of ordinary finishing on any pour. It is one of the small things that separates a slab that holds its finish for years from one that starts flaking or dusting within the first season.
What makes more of it show up
Some pours produce more laitance than others, and it usually comes down to how much water was in the mix and how the surface was worked. A wetter mix bleeds more water, and more bleed water carries more fines up with it. Overworking the surface with a float or trowel while bleed water is still rising does the same thing, pulling fines up faster than the slab would have brought them on its own.
None of that means a particular pour did something wrong. It is a normal part of how fresh concrete behaves, and finishing it right is about reading the slab and working it at the right stage, not avoiding the process altogether.