Rest API and client side storage

Thanks for the support, I have been going through them because I have no idea in app building but working on one. Please could you clarify, the client side storage vs the Rest Api. I am currently using the client side storage but I cant use output from another node when using set app variable. Does output from another node only apply when using Rest API?
Also must I use only the client side storage or I need to use REST Api as well for my data storage/backend. Thanks