sometimes i don´t feel that i need to use "için" and sometimes i forget it maybe because i don´t know its usage.

for example in beginner 4 class:

Emre: Akşam fen bilgisi ödevini birlikte yapacağız, değil mi?

Murat: Evet, onun için defterimi de getireceğim.

would it differ if Murat said his sentence without "için"?

Thanks alot for your help

Murat shortly can say "Defterimi de getireceğim" (I´ll bring my notebook).

But we may think, why?

"Onun için" For that (reason).

"that" is in the first sentence, they will do the science homework together.

çoooook teşekkür ederim İkicihan