This is an old practice in Cricket. Prior to 1970s, it was v common for fast-medium bowlers to have the WK standing up unless they were express (over 135-140 kmph)
A rarity in our times
In fact many claimed that Bedser was a much much better bowler when the WK stood up than when the WK stood back
It focuses the bowler's line and length. Forces him to bowl straighter, and not bowl too wide or too short
Second it is a signal to the batsman not to charge down the wicket, given the possibility of a stumping