The UK parliament knows terms (roughly the time between two elections) but primarily deals in sessions. These sessions begin by the Queen opening parliament with the Queen’s speech and invariably end with prorogation. At the end of a session, all bills that have not been passed are considered abandoned. Customarily, a session will last approximately one year but the current session has been going on since the 2017 snap election so it was ‘about time’ for the session to finally end.
It is not necessary for the Johnson government to have a Queen’s Speech and State Opening of Parliament to present new legislation. However, the Queen’s Speech is an important date in the UK parliament year as it presents the government’s agenda for the upcoming year. It won’t be the presentation of legislation in and of itself but it will be a programmatic speech of what is to come. Since Johnson only recently became Prime Minister and since most of what he has had to talk about concerned Brexit, this speech will give the Johnson government a first chance to formally outline what else it plans.
It is also not necessary for the State Opening of Parliament to be held at any certain time. It could have been held in November if Johnson had considered that the better date. It is also not necessary for the relatively long period of prorogation preceding the State Opening; this could probably be as short as a day (and indeed it seems there was a 1 day prorogation period in 1948).