Not all or every ISP or Internet Service Providers have data caps and if they do most people will never even go over or get near the data cap.
The data caps that some ISP companies have in place is to prevent excess use and strain on their service so that other peoples internet service doesn't get slowed down etc.
Some people download a lot of files, games, movies etc and that takes up bandwidth on the main internet line which causes traffic congestion so other peoples internet will get slowed down.
The ISP also puts in place Internet data caps to make them more money because if the user is gonna use more internet then they should be willing to pay.
The ISP wants to milk all the money they can out of you to make them richer and keep the service going.