It is also used for synchronization. 

Respected sir,

Thank you for your answers. 

Shall i know how a pilot tone helps us in identifying whether a channel is used by a subscriber or not?Please refer me some material to understand this. 

Time and frequency positions and gains of the pilot tones are known from both transmitter and receiver. Therefore, it can be used to implement simple detectors: energy, matched-filter etc. at the receiver. May I suggest this paper:

http://users.ece.utexas.edu/~jandrews/ee381k/EE381KTA/article16.pdf

for more details.

Thank you very much sir,

pilote tones are also used to detect channel gains for every sub-carriers, it helps for adaptive modulation or bit, power allocation.

Ref:

hhttp://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=793310