What a sealer is standing between
Bare concrete is porous, so water, oil, fertilizer and de-icing salt all have some ability to soak in rather than sit on top. A sealer closes that door to one degree or another, either by lining the pores or by laying a thin protective layer over the surface. What it is not is a fix for a slab that already has a problem. Sealer protects a sound surface. It does not stop a crack, fill a settled spot, or undo staining that has already worked its way in.
Putting it on before the slab is ready
New concrete keeps releasing moisture for weeks after it looks dry, part of the same curing reaction that builds its strength. Sealer applied too soon traps that moisture underneath it instead of letting it escape, working against the slab rather than for it. The fix is patience, not a different product. A slab tells you when it is ready by how it has cured, not by the calendar, and that timing depends on the mix and the weather it went down in.
Not every slab needs one, and that is fine
A broom finish already sheds water reasonably well on its own, which is why so many driveways and walks go their whole life unsealed without any trouble. The slabs that genuinely benefit are the exposed ones and the decorative ones. Stamped and colored work heads that list, along with any floor where a stain would be the thing you notice every time you walk past it. Sealing everything by default is not the goal. Sealing the surfaces that actually benefit from it is.
Letting it wear out and not replacing it
A sealer is a wear layer, and wear layers wear. Traffic, sun and weather break it down over time, and once it is gone the slab underneath is back to absorbing whatever lands on it. Left alone long enough, especially on decorative work, the fix stops being a simple recoat and turns into stripping and rebuilding the surface, a bigger job than staying ahead of it would have been. Applying it wrong causes its own problems too: sealing over a surface that is not fully dry can trap moisture and leave a milky haze, and one heavy coat tends to peel where two thin coats would have held.