MarketPress and Pro Sites... Integration

Hi,

I read in the description you can extend Pro Sites with other plugins including MarketPress. I am not however sure what this means.

"You can also extend Pro Sites even further by integrating it with many of our popular plugins, including:
MarketPress"

Can someone shed some light on what this means? What it can do? Does it open up more gateways for example.

For my site, I would like to sell add on services to my customers outside of their monthly subscription.

Cheers