I know how to make attribute mandatory, but please guide, how to make value /descriptive field mandatory.

I do agree that it may seem silly that you can make the attribute required and people can leave the description field empty but it depends on how you are using attributes as to whether or not a description is necessary. 

The required checkbox is obviously only making the category required.  If you want the description field to be able to be marked required create a case with Blackbaud and also add the idea to the ideas section here. I would add my vote to it. It may or may not make a RE7 update but could very likely be incorporated in some way to RE8.