Track car windows?

[Brooky90]

Was at Castle Combe circuit a few weeks back watching track cars go round the track and drifting etc. It got me wondering what's the purpose of the little square window on the track cars? These track cars always seem to have a small square shaped open window in line with the side of the driver's face, what's the purpose of this? I don't really know anything about track driving so someone enlighten me.