what is the use of boolean operations in catia? i known how to create but i don't know what is the use for it further ?

It's on the top left. Slide down to third option. Move to the second sub-option. What you require should be within your program.

Here is a well thought out tutorial:

http://www.computeraideddesignguide.com/boolean-operations-in-catia/

Boolean operations were required back in CATIA V4's part design. But with V5 (and V6), the new solids modeling techniques no longer require using Booleans to model most parts.

Only special situations, where multi-body solids are used, require using Boolean operations.

Of course, when a V4 model is converted to V5, the Booleans will be transferred and found throughout the model.