Maybe he made them via Herblore and they couldn't sell so he just uses them.
Other than that, I have no idea why he wouldn't just use a Super Att and Super Str
If your DH training, its a good way to get lower HP.
If your training at PC for example, just take a couple doses, your HP+Def lower so a monsters have a easier time hitting you. After that, just drink a regular stat restore pot.
To elaborate on what 333 said, they're used to lower your hp while training with dharok's.
As dharok's hits harder depending on how low your hp is, zammy brews are a low-risk way of getting low hp.
Then you can go to ape atoll monkeys or something and train with your prayers on and without risk of getting hit. It will increase your average hit by a lot, and will consequently raise your xp/hr.