Why are there grammatical categories like number, person, gender, tense, aspect, voice, case, mood, comparison in languages ?

Answer 1

Because they're accurate descriptions of the ways in which words are used.

Answer 2

Possibly because some people are interested in whether an action is in the past, present or future, in whether a person is male or female, in what the quantity of a group of items is, in whether one thing is bigger than another, and other insignificant details that you find uninteresting.

Answer 3

They serve to nuance communication.  Without categories like this any language would be flat and limited.

Answer 4

It's all about communicability and intelligibility!

Answer 5

Because the original speakers also found that they could communicate more quickly with other native language speakers by using most of these things.  Lots of times speed (as well as subtlety) in getting an idea across is also very important. 

Answer 6

Because the original speakers of those languages found they helped them to convey much more subtle meaning than by NOT using them.

Answer 7

For wasting the time

Answer 8

So people can communicate without the need to draw stick figures and make grunting noises.

Answer 9

because it works