Imagine you're at a restaurant. The waiter asks what you want to order.

"I want a glass of water," leaves open the possibility of wanting other things. The waiter will wait for more information.

"I just want a glass of water," indicates a desire for water exclusively. The waiter will serve you a glass of water and hand you the bill.

Two completely different meanings and outcomes, differentiated entirely by the word "just".

There, I just told you two good ways to use the word.