We can catch simple bugs like GUI ,small functional Bugs 
during unit testing.This reduces testing time.Overall this 
saves project time.If developer doesn't catch this type of 
bugs,this will come to integration testing part and if it 
catches by a tester,this need to go through a Buglife cycle 
and consumes a lot of time.

or 

Unit testing helps in verfying that a unit or module that 
is built is as per requirements and functions properly .It 
helps in preventing bugs getting multiplied and migrated in 
later phases.