The point is that the (presumably public or school) network I am connected to will have a hard time blocking any content for me (given it will only keep seeing packets between the phone in my hand and the router at my apartment). It will also encrypt my traffic, so MITMing me or trying to see what I am doing will become rather difficult even for the operator of the network I am connected to.
Edit: I think I just realized what made you confused in my original comment. I didn't specify explicitly, but it was implied that I toggle VPN on my phone when I am outside of my home and am connected to a work/public/etc network. Because otherwise, you are correct, trying to VPN through my home network while my phone is actually at home already connected to that network will serve no purpose at all.
Plenty of reasons. Many people need a VPN to either bypass network restrictions (like this) or add a bit more security to an untrusted network (eg, cafe wifi). Setting up your own VPN is free, and in some cases, more secure than a shady VPN provider.
