Soldering wires, specifically. Wires are flexible, and solder joints are not. If the joint tries to flex anyway, it can crack. If it cracks then it no longer conducts so well, and instead generates heat. That's both wasteful, and a fire hazard.

When attaching wires to things which needs to be robust and reliable, you generally see crimp connections of one kind or another, which are much less susceptible to damage because of wiggling around.

On a PCB is another matter, because the board is already rigid and unbending so using a rigid joint is fine