There's a couple reasons you might want to run a relay on your phone.

First is just to back up your own notes. It has particularly come in handy if I need to restore my follow list because some random Nostr app I tried out nuked it.

It can also be used as a private local relay for just your drafts and eCash tokens.

Finally, you can use it with Orbot to have it accessible over Tor and use it as your DM inbox relay for truly peer-to-peer DMs.