Why are conformational isomers considered as isomers?
Gaurang's answer deals well with the theory part I just wanted to give a visual aid.




(Taken from Atropisomerism of naphthyl alcohol)

Let's call the rotating substituent on the naphtahlene a fan.
The fan has two black fins one red fin.  
Now let us consider two conformations


  Red fin at 3'o clock


And 


  Red fin at 9'o clock


If you were to take snapshots of both the conformations they look different.
Now these two isomers are different and naming them as 9'o clock and 3'o clock  is obviously very inefficient.
That's why conformational isomers and their nomenclature exists, for us to point out specific configurations of a compound.