So in the same way that John Diggle, Roy Harper, and Malcolm Merlyn have all donned Oliver Queen’s Green Arrow suit over the years, Wally may actually be posing as Barry to convince Central City’s citizens that there’s nothing to worry about, in the face of crime and the metahuman threat.
With all of that said, there might be another much simpler explanation for why Wally West is wearing Barry’s suit in this set picture.
Central City (among other locations) believes in The Flash as a symbol of hope, and criminals could declare open season on the city if they found out The Scarlet Speedster was no longer running around (even if heroes like Kid Flash and Jesse Quick are around to help out).