If your slab is cracked, heaved, or holding water in low spots
Look at what your old slab is actually doing before you assume a new layer solves it. A crack that runs in a straight line at a joint, stays tight, and does not step up on one side is cosmetic and the slab underneath is still structurally sound. A crack that has a lip you can feel with your foot, a corner that has dropped, or a section that holds water after every rain is telling you the ground under that slab is still moving.
That difference is what decides whether an overlay is even on the table. It is not a matter of how the old concrete looks on top. It is a matter of whether it is still stable.
Why a new layer does not fix a moving base
An overlay bonds to the surface it goes on. If that surface is stable, the overlay is stable with it. If the old slab is still settling or heaving because the base underneath was never right, the new concrete rides along with every bit of that movement, and the same crack pattern shows up again on top, usually faster than it took the first time. Pouring over a problem does not remove the problem, it just adds a layer on top of it.
That is also why a thin cosmetic resurfacer is a different thing from a structural overlay. A resurfacer refreshes the look of sound concrete. It is not built to carry a load on its own or to bridge a slab that has real structural movement in it.
When an overlay is the right call
On a slab that is flat, sound, and stable, but worn, stained, or just dated looking, an overlay can make sense. We tie the new layer to the old one properly rather than just laying concrete on top and hoping it sticks, and we plan the edges so the new surface meets doors, steps and adjoining slabs at the right height instead of creating a lip somewhere you did not expect one.
When tear-out is the honest answer
If the old slab is cracked through, heaved, holding water, or thin and crumbling at the edges, we would rather tell you that plainly than sell you an overlay that fails in a season or two. Tear-out means the old concrete comes out, the base gets rebuilt properly underneath, and the new slab goes in on ground that will actually hold it, sometimes with dowels set into an adjoining slab so the two pieces move together at the joint instead of stepping apart from each other. It costs more up front than pouring over what is already there. It is also the version that lasts.