Where concrete's own stone gets in the way
Regular concrete carries coarse aggregate because stone is what gives a slab or a footing its bulk strength cheaply. That same stone is a liability once the space you are filling gets narrow: a bolt sleeve, a void behind a form, a gap under a precast step, the core of a block. Stone that size bridges an opening that width, and the load stacks up behind it the same way it does inside a pump hose that is too small for the aggregate going through it.
Take the coarse aggregate out and drop the sand-to-cement ratio toward the fine end, and what is left is a mix that can be worked into a space with a trowel, poured from a bucket, or in some cases pumped through small line, then hardens with the space filled solid rather than bridged.
When the wrong load ends up in the void
The trouble usually starts because grout is not what is sitting in the truck. A crew mid pour reaches for the same load that is already going into the slab, because ordering a second small batch feels like an extra step for a small void. The stone in that load does exactly what stone does in a narrow space, it bridges, and the gap behind it never fills. What looks finished from the outside is hollow behind the surface, and that hollow spot is the kind of thing that shows up later as a wobble under a step or a plate that was never really bedded.
When grout gets stretched to carry a slab's load
The mistake runs the other way too. Grout is carrying almost no stone, so it does not have the bulk compressive capacity a footing, a slab section or anything load bearing needs across a wide area. Using it to save a trip to the batch plant on real structural work leaves that patch weaker than the concrete around it, and because it is mostly paste and fine sand it shrinks more on its way to setting, so it is also the more likely of the two to crack on its own schedule.
The void has to be clean before either mix goes in. Dust, loose material or standing water left in the space breaks the bond between the grout and what surrounds it, and a patch that never bonded pops loose later even when the mix itself was the right one for the job.