Which mode is being used?
If I remember correctly when VNAV is being used the bug automatically reduces to the speed when the new setting is made on the flap handle, so I don’t see a reason for changing it when using FLCH or some other mode.
Probably somebody with a bored brain that too big for its own good flexing their ego, on the other hand it’s possible that there might be a good reason for doing so that I’m not aware of.