I'm pretty sure it is to enable engine bleed operations after the start cycle, with the crew not having to make another selection.
That is, when engine start is initiated and the start valve opens the Bleed Air Regulator vents control air from the Bleed Valve - PRSOV and it closes under its spring force pressure. When the start valve closes the BAR solenoid allows control air to open the PRSOV ( this can only happen if the ENG BLEED switch is ON ).
Therefore, had the Bleed switches been OFF, the crew would have to make those selections manually to enable engine bleed air.