Quite well question my brother,
Read only domain controller are very significance when you want security,accessibility and reliability of your network.
here below example will explain how RODC significance to you,

A sales person work in big hospital and as a user of domain he can access all faculties as pr given by managements but some how he is not able to come office (Hospital) and managements want he continues his work from home.
then we can make RODC at his home and allow him to access our domain login from his home. 

and have more example, i don't think i need to explain you because you are smart person.
...................................................................................................................................................................................................................................