Do I need a brush insert, or can cables pass through the normal opening?

You don’t need a brush insert.

Cables can pass through the normal opening once the cover has been removed. A brush insert gives the entry a tidier finish and reduces the amount of completely open space around the cables.

It can make a populated cabinet look rather less as though somebody removed a steel plate and then forgot why.

Use one where the cleaner cable entry is worthwhile, but it isn’t required for the cabinet to function.