Noob question guys regarding the use of rear view mirror.

Aside from backing, where and what's specifically the use of it?

Nung nag aral ako sa driving school, ang sabi ni instructor is, it is use pag nag overtake ka, you need to look at it instead of the right side mirror to make sure na malayo ka sa na overtakan mong sasakyan bago ka bumalik sa right lane. Though for me, mas sanay ako tumingin sa right side mirror instead of the rear view mirrow before ako bumalik sa right lane.

Is it a bad driving habit?