Question

why god allah sent a prophet???????

Answer 1

To guide human to the right path

Answer 2

Look Allah SWT didn't sent just one prophet. He sent 1lac 24thousand prophets at different times when humanity was getting into bad stuff.
When Nabi Muhammad (Peace be upon him) was sent on Earth at that time The Arabs were burying their daughters as soon as they were born and were committing adultry.
So Nabi was sent to guide them to the right path.

Noctua @_@