Yeah, it's a weird rule, as it usually makes a lot more sense for the enemies to do it: if your party of 4 is facing 20 zombies and one grabs your mage, you're fucked. But grabbing a zombie with your warrior is kinda pointless.
In this module it might be useful to stop a caster that can easily fuck things up with a single spell and can't be easily killed.
Yeah it's quite situational, but grapple is a useful tool in the martial toolkit. One that people often forget about.
If you apply it well it can totally change the dynamic of a fight that's going poorly.