To my opinion, nothing. You can even use different account names with the same Home Pro licence. Just if you do this, the devices won't be "synchronized automatically". But you can still add the folders manually. Their is no control on the number of seats or the user name. However, ethically, it’s better to buy a Family version if you use this licence on computers of other family members or friends. It’s a way to thank them for the great they’ve done in this software development. That’s why I enjoyed their last offer and upgraded my licence to family version (an extra $10 really worth this software).
So again, from the info I could get from the Resilio Team, I would say their is no technical interest to switch from Home Pro to Home Family Pro, unless you want to thank the great work they’ve done on developing such great software.
But again, it's only what I could conclude from what I read, and it's succeptible to change in the future if they start to control the number of seats or user names.