What it soaks into, and what it leaves alone
Concrete is porous even when it looks solid. A penetrating sealer moves into those pores at the surface and lines them, cutting down how much water, road salt and oil the slab absorbs. Because it never sits on top as a coating, it does not add gloss, does not build a film that can peel, and does not change the texture or the slip resistance of the surface underfoot.
That is the whole trade-off in one sentence. A topical, film-forming product changes how a slab looks, and a penetrating sealer changes how it absorbs, without touching the look at all.
Putting it over a surface it cannot get into
A penetrating sealer only works if it can reach the concrete underneath it. Applied over a slab that already carries a film-forming sealer, a cure and seal coating, or an epoxy, it has nowhere to soak into. Most of it sits on top of whatever is already there instead of doing the job it is built for, and the result is wasted product and an uneven, sometimes blotchy surface rather than the protection anyone paid for.
The same problem shows up on a slab that was not cleaned first. Dust, grease film or an old failed coating left on the surface blocks the pores just as effectively as a new coating would, so a penetrating sealer applied over a dirty slab often ends up doing very little.
Expecting the look a topical sealer gives
People sometimes choose a penetrating sealer expecting the darkened, wet look that a film-forming product gives stamped or decorative work, then wonder why the slab looks the same as it did before. That look comes from a coating sitting on the surface and catching light differently, which is exactly the part a penetrating sealer skips. If the goal is protection with no visible change, that is what it is for. If the goal is a richer color or a glossy finish, a penetrating sealer will not deliver it.