Hello Aasif Bashir
As far as Bessel filter is concerned i agree with most of the responses. Undoubtedly it is used as a default filter for most of the optisystem projects as Karan has mentioned. The reason may be that the Bessel filter is a linear form of filter that provides a maximally flat group delay or propagation delay across the frequency spectrum, but offers a slower transition from pass-band to stop-band than for other forms of filter of the same order.
Also i agree that it has a maximally flat group delay means that Bessel filters are often used in audio applications such as audio cross-over networks as mentioned by other forum members.
I believe it is application specific. Hope this is helpful to you.
Regards
Burhan