ans
1.
    sinusoidal functions are easy to analysids
2.
any non sinusoidal function can be transformed into 
sinusoidal functions according to FOURIER THEOREM

3. in especially Electrical engineerig  we r using
mainly the supply as sinusoidal otherthan non-sinsoidal
because  all elctrical circuits have only r,L,C as passive 
elements.for finding current in a inductor for a given 
sinusoidal a.c supply we have to calculate through 
integration of sinusoidal input which gives again as 
sinusoidal.however, with some phase isplacement.Simmillarly 
for capaxcitor also to find current from sinusoidal input 
voltage we require differentiaton, which gives again as 
sinusoidal.however with some phase displacement

non sinusoidal waveforms contain harmonics which may damage 
the devices

1.Sinusodial signals have only one freq component which is 
its fundamental freq.Every system's gain is calculated with 
reference to sine signal only.THE reason being every system 
is bound to have a finite bandwidth and if use other 
signals like square for calculation of gain ,it would give 
errorneous rersults, as squre has infinite freq comp and 
higher freq comp will not be allowed(Due to finite bw of 
s/m).
Hence as gain is wrt to sine then it makes sense to use
sine signal only.

2.It doesn't hav dc comp and hence helps in coupling with 
transformers.