Other than bugs and road dirt, the primary function of a 'bra' is to keep droplets of water from driving into every nook and cranny of the bike while you drive at 60 MPH in the rain. Even 'sealed' bearings can be at risk of ingesting water in those conditions.
If you are going on a trip with the bike outside of the car and want to protect it you can use pieces of old tubes, plastic bags and tape to the same effect.

Enjoy