Cell walls prevent plant cells from bursting.
Blow up a balloon until it pops. An animal cell (or a plant cell that's missing a cell wall for some reason) is like this balloon. Water can flow into the cell until the membranes bursts.
Now place a balloon into a length of pantyhose and proceed to blow it up. It will be harder and harder to blow up the balloon because the nylon restricts the balloon. Virtually impossible to blow it up enough to pop it. In the same way, the cell wall prevents the cell from reaching its bursting point.