The side brush covers a narrow strip maybe half an inch along the side still present with the
Botvac, which did bring the brush end closer to the side than the original XV model (hence the side brush on the old Vorwerk version). The bumper structure on the Neato prevents the brush end being brought out all the way, needing some room for the bumper action, and would scrape the wall unacceptably if closer. Dyson has made a thing about getting the brush end out far.
I am not a fan of side brushes but opinions vary. Their addition to the first
Botvac's was welcome by many users here and they can add their side to the story. Yet plenty of posts here about them falling off, not surprising with obstacles, and some types I have had break against sharp edges of bookcases and what not. I also found some of them at least, pull on wires placed against the wall to escape devouring robots, requiring more fussy less flexible installations of furniture. They are delicate and wear out (replacement packs of them sold). They do not work on carpet only delicately sweeping the surface. The side brushes on round bots help a lot in corners.
The belts for side brushes wear out over years and one more motor is subject to contamination with dust.
The side brush is not the only additional feature on more expensive bots. Wider intakes, larger batteries, more powerful suction, whatever. WiFi apps are an expensive recent addition, with very little use as far as I can see, with the minimal functions supported so far.
Expensive Samsung Powerbots have no side brushes. Fine with me, but some users are more fussy.