Paint first, or cutout first?

[feasty33]

Hi,

I'm not that new to painting, but I'm definitely new to painting properly!

I just have 2 questions both regarding 1/10th TC shells.

Firstly, why do the pro's cutout the shell only once its been painted?
I used to assume it was to help minimize overspray getting onto the outside of the body, but with the protective shrinkwrap now fitted I'm not so sure.

Secondly, what is the benefit of masking out the centre of the wheel arch area?

Thanks in advance for any answers

Dave