Paint extender like Floetrol does two things :

It reduces the paint drying time, giving you a longer window period to overlap just painted areas without having lap marks left on the walls.
Secondly, the extender helps paint to levels out and spread out the paint, eliminating the brush strokes.Paint extender is usually used when you have to color woodwork, cabinets, and drywall and so on.   _{Your Comment}