High fat in the diet will contribute to diarrhea. So the immediate effect of lowering the fat content will help get the diarrhea under control.

The long term reason for doing so is maybe your Vet is starting to suspicion Pancreatitis. High fat in the diet aggrevates the Pancreatitis if present.

Another possibility is Inflammatory Bowel Disease (IBD), where low fat, and high fiber can help.

Curiously enough Metronidazole has some beneficiary effect for both Pancreatitis and IBD. Sounds like your Vet has a good plan.