There are probably different types out there, but the type I used is applied to a cured slab.

These are all good.

https://www.legacyindustrial.net/products/densifiers/

It's used in alot of different ways. Sometimes applied before the slab is put to to work. Other times between stages of grinding and polishing.

It's well worth it, even if you change your mind and apply a coating later....but it prevents spills of all kinds from soaking in.

If you do a coating later on....and the coating ever fails you won't have dust and so on. If the slab has to be ground first before applying the coating, the densifier will give a nicer substrate.

I couldn't think of a reason to not use densifier, other than a slight expense. It's super easy to apply.