what is the basic reason for reverse engineering

Reverse engineering is often done to generate engineering documentation for existing products for verification, authorisation and quality purposes.
It can also be done to copy someone else’s design.
Some reverse engineering activity may be required so that purchased components and equipment can be used in higher assemblies that are being designed.

I think there are many reasons but you get good practice modeling, learn how something works(in great detail) and it can make improving an existing idea easier.

thanks.......

Basically when one person is trying to base a design or manufacture something from a competitor or another country. China pretty much every thing that is made is reverse engineered.