When to use Javascript or Backend Languages? [on hold]

I just finished my first website and realized I used very little Javascript. This is mostly due to me not knowing much JS but I'm wondering what advantages does JS have over backend processes?

Of course some things must be done client side but a lot does not.

Would using excessive backend languages cost a lot of processing time (money) if the website was large in size? Example being html forms, does using a client side language give an advantage over doing all the form processing backend?

I'm starting my next site and would love to know what type of front/back balancing I should be trying to achieve.