A primer decides whether the system bonds. A sealer decides how the finished surface looks, how easily it cleans and how long the colour holds. Both are chosen for the substrate and the exposure, not by habit.
Dusty or porous surfaces take primer first. Dense, sealed slabs are ground open instead.
A pore filler closes a stone carpet for easier cleaning; leaving it open keeps the drainage.
Gloss deepens the colour and shows wear sooner. Matte hides traffic and dust.
Describe the surface and what it will be exposed to. We name the primer, the sealer and the quantity, and say when none is needed.