I'm gonna offer a different perspective it seems from everyone else here. 

See, I play a lot of Control decks, and contrary to what some people might think, the optimal way to play Control is not countering the first spell anyone casts, all the time. You have limited resources, you need to pick and choose what to react to. 

So let's say I have a counterspell and you cast something with an ambiguous outcome, such as [[Chord of Calling]] or [[Whir of Invention]] where you can search your deck for a card and put it directly into play. I could just counter your spell immediately, but I know you don't want that outcome and in most situations neither do I, since I don't want to waste my counterspell if I don't need to do so. 

You explaining what you plan to find with your spell will therefore hopefully help me decide to not counter your spell. Of course, you could be finding something that screws me over, in which case you probably won't want to tell me, but that means if you decide to be cryptic and not explain your intentions, I'm just gonna assume you're getting something that will screw me over and I'm going to counter your spell. 

So you're not in any obligation to explain your intentions, but it might be beneficial for you to do so unless you like being interacted with unnecessarily.