Let me give u a example. I log into a school account which needs me to log in . On hand I have other devices, e.g tablet , Android TV etc. Some of these may not be able to log in the wifi using the same account.
But I can share my wifi to these other devices. They simply have to log into my phone wifi.
Another example, I was on holiday in a kelong Malaysia. The host place the router too far from the room. I can put my phone in the middle as a repeater, from the room I could reach the wifi with my other devices.
Sent from HUAWEI CLT-L29 using GAGT