Holding cracks together, not holding a building up
Concrete shrinks slightly as it cures and dries, and that shrinkage is normal, expected, and not stoppable by any steel in the slab. Wire mesh does not prevent a crack from forming. What it does is span across the spot where a crack starts, so the two sides stay tied to each other and the crack stays a tight, fine line instead of opening into a gap that walks apart under traffic.
That is a different job from what rebar does on structural work, carrying real load in tension. Mesh is sized and used for crack control on thin, unloaded flatwork, and it is not a substitute for the reinforcing steel a footing, a foundation wall, or anything carrying a building calls for.
What goes wrong when the mesh never leaves the ground
The most common way wire mesh fails to do its job has nothing to do with the mesh itself. It is rolled out flat on the compacted base before the pour, and if nobody pulls it up as concrete goes over it, it ends up sitting at the very bottom of the slab once the concrete cures. Mesh at the bottom of a slab is not doing anything, because a crack starts at the top surface and works down, and steel that is not up where the crack is cannot hold it together.
Seams matter too. Sheets or rolled sections need to overlap enough at the edges to act as one continuous grid rather than two separate pieces that happen to be next to each other. A seam that is not lapped and tied leaves a gap in the coverage right at that line, and that is exactly where a crack finds the path of least resistance.
Where mesh is the right steel, and where the plan calls for something else
Wire mesh suits thin flatwork, a walkway, a thin patio, a decorative overlay, where shrinkage cracking is the whole concern and nothing structural is riding on the slab. On structural concrete, a footing, a foundation wall, anything carrying a wall or a roof, the plan specifies bar size, spacing and lap lengths, and that specified steel is what gets placed rather than mesh standing in for it.
Fiber mesh is a different answer to the same shrinkage question, tiny synthetic fibers mixed straight into the concrete rather than laid as a sheet, and that is a call made ahead of the pour rather than a substitute decided on site. Our fiber mesh glossary entry covers that comparison.