Why does programmer need float or integer? In what situation they decide for using float or integer?

Answer 1

Numbers are extremely important for keeping track of the program flow, and make calculations. There are an insane number of uses for them, although most of your programs will probably use a lot of integers, and not so many floating point numbers. Another very important thing:
Numbers are universal, language isn't. Using message numbers instead of message literals is a great practice when working on production code.