Prying quality directly determines how likely you are to pry successfully, along with Strength. A makeshift crowbar (with a pry of 2, I think) is less likely to pry successfully compared to a Halligan bar (with a pry of 4.) With windows, it also determines your chance to pry it open instead of shattering it; a low-Strength character with a poor tool will break windows a lot more often than a high-Strength character with a good tool.
I don’t think safes can be pried open, but I haven’t ever tried.