By flowing through the aspirator, water will suck out the air contained in the vacuum flask and the Büchner flask. There is therefore a difference in pressure between the exterior and the interior of the flasks : the contents of the Büchner funnel are sucked towards the vacuum flask. The filter, which is placed at the bottom of the Büchner funnel, separates the solids from the liquids.
The solid residue, which remains at the top of the Büchner funnel, is therefore recovered more efficiently: it is much drier than it would be with a simple filtration.
The rubber conical seal ensures the apparatus is hermetically closed, preventing the passage of air between the Büchner funnel and the vacuum flask. It maintains the vacuum in the apparatus and also avoids physical points of stress (glass against glass.)