Here's a convenient example.
Suppose you are trying to solo Confusion Alakazam. At level 40, Gengar has a defensive breakpoint with 15 def IV and 13+ HP IV that allows it to survive 4 Confusion.
If it doesn't have 15 def IV, then it faints in 4 attacks. And it's very likely that Alakazam can use Confusion 4 times in a row against Gengar.
Against other raid bosses, they will use some combination of fast and charge moves, so this is not as predictable, but there are definitely instances where lower def IV means you're getting in fewer attacks or maybe even 1 less charge move.
