Does a consultant add real value to what the corporate already know, when he is engaged to solve a problem or suggest solutions ?

When it's a good consultant, yes.  I have advised many companies on how to solve a problem which the company was not able to solve by itself.  In fact it is my speciality.*

Successful corporates tend to be good at solving common or familiar problems, but can have trouble with situations which the company has not faced before. 

* If your company, or somebody you know, is struggling with a really tricky problem please get in touch.  I'd be glad to help.