Should I Use Core Data or not?

I'm developing an application for the iPhone which gets all data from a database from a web service. I receive the information in JSON format and make constant calls to the web service for data. My question is should I be using core data? Right now I just make a request to the web service use the information I need and make calls to data when needed. But should I put the data into core data because I cannot figure if it would help or even make sense to do. Since there are thousands of rows of data on the server I can't imagine constantly saving information into core data in effect copying the database from the server onto the phone. That does not seem to make sense or be a good idea performance wise. Wouldn't I eventually run out of room in the core data database or use up too much room on the device? Maybe I'm not fully grasping the concept.