How is learning about psychology useful? what needs to happen for it to be useful?

Psychology is the study of mind and behaviour, learning it is useful for a number of reasons. One of them could be your day to day life, interacting with close friends and family, learning how to understand where their opinion is coming from and what is usually going through their mind when coping through loss and grief, for example.
It can be used as the core of your future career, no matter where you live, you can always do a double degree of psychology and your biggest interest, for example:
Sport and Psychology: focuses on athlete's recovery and mental fitness
Forensic Psychology: focuses on criminal profiling and police work 
and so on...