How something such collective and homogeneous like The Collective could need, create and use something such independent as queen, a single drone to rule them all?
The single answer given by the Borg Queen during Star Trek: First Contact is:
BORG QUEEN: I bring order to chaos.
Given that we know the Borg do start to regain individuality when separated from the Collective for too long, it is possible that a connection directly to her is required to maintain the suppression of individuality. If it is so, she is the one making the determining call when the possibility of discord arises; in a situation where the Borg could come to an impasse, or analyze the same situation differently, her direction is a requisite to redirect the hive mind away from what would otherwise be a dead-lock.
Unfortunately there is little actual data to back this up, save her own odd statement on the subject.
There is no conclusive answer for this, unfortunately. The Borg Queen is still a matter of some debate and disagreement among Trek circles, and no definitive answer has ever been given as to how or why the Queen exists. There are several non-canon works that deal with the Queen, but they all contradict one another, therefore leaving us with no accepted answer as to how the Collective can even possess a Queen.
In the Star Trek destiny crossover books based in tng, aventine, Titan and ent, we find the Borg queen is originally a member of the Cealiar, a hyper advanced species that almost gets destroyed by macos from USS Columbia, that has merged with a human being.
The reveal comes best the end of book 3:
A roar of voices spoke the will of the Borg.
You will be assimilated. Your diversity and technology will be adapted to service us. Resistance is futile.
The gestalt was overwhelmed with pity for the primitive and autocratic posturing of the collective...
...Edrin the architect asked Do we know who it is?
It is time we found out said Ordemo.
[They] focused the gestalts attention on breaking through the noise of the Collective, penetrating to the true essence of the Borg, exposing its prime mover , revealing the mind at ya foundation and the voice behind its queen...
...It had no memories of its own, no name beyond Borg, but as the gestalt took its full measure, it was recognised by one and all for what and who it truly was.
Sedin said Inyx, baring his grief for what had become of his confident and beloved companion of several aeons...
...Once she had been a Cealiar Scientist and poet.
Cealiar are xenophobic isolationists millennia old sustained by genetically engineering themselves to use nano machines called catoms which keep them alive and are used for all their tasks and construction.
They have superior technology and when the macos tried to escape ended up destroying one of their cities. Another civilisation that we find out in time is their own sends an interference signal to their 'great work' destroying the planet and scattering the Cealiar survivors across space and time. Some end up at the dawn of time. Some end up in the delta quadrant notably Sedin and some crew of Columbia.
The inception of the Borg is quite creepy from book 2 with the last words of the first human that causes the corruption of the Cealiar catoms of Sedin being :
"NO! I don't want to become a Cy"-'Borg'
Thus the first thought of the first drone was the name Borg.