Holley instructions contradicting on fuel bowl level ??

Also, if you run an electric pump, make sure the engine is running when you set the float level.

Why do you do that? Just wondering as I thought a nice perc of an electric pump was the ability to set floats on a still motor..? Its a messy job when its idling if your new to it (like me) fuel squirting everywhere! Always seemed to have Carters or 'brocks where you could set the float level properly when its apart.