In a vacuum it creates congestion, in reality things can be different, usually shitty drivers

For example: I drove a large truck most of last year and people simply will not let you in if you attempt to zipper mete properly in a vehicle deemed slow or cumbersome. They will literally close in as tight as they can on each other knowing that should you hit them you'll be found at fault

This can even result in the people in the lane behind said vehicle to start zippering at the point behind the vehicle, essentially moving the zipper a car length down the highway. These people also frequently close in and box out that cumbersome vehicle as well.

This technically keeps traffic flowing, but massively breaks flow when the vehicle finally managed to edge enough into the lane to force a merge or someone gives them room to actually get in