As I understand it (and I'm not expert), Parliament was going to be suspended again on 14th September anyway, for party conference season.
There is a big difference between suspending Parliament and proroguing it. The former happens allk the time (they've just had their sumemr break), and parliamentary buisiness gets picked up again afterwards. Prorogation entails ending teh current session of parliament, dropping any legislation that is underway into /dev/null and starting again after a new Queen's Speech. That essentially means that any business Parliament brings before the prorogation date has to be concluded or it doesn't happen, and that the first week or so after the new Parliament starts is taken up with debating the Queen's Speech and no ohter business. The timing and length of prorogation is pretty obviously calculated to make it as hard as possible for Parliament to do anything to stop Johnson doing whatever he likes about quitting the EU with no debate or oversight. Whether you agree with leaving or not, it's a pretty clear attempt to sideline parliamentary democracy in favour of autocracy.