Why does the Ultimaker 3D Printer has a Heater + Heater transfer plate (aluminium) + Glass?
According to this page, heat transfers more evenly across an aluminum build plate than with just glass. But as Kevin pointed PLA adheres better to glass because it doesn't flex as much as aluminum under heat.

The link above shows that aluminum has a much higher thermal conductivity at 205 (W/(mK)) vs glass at 105 (W/(mK)) at 25 &deg;C (77 &deg;F).

Because of this conductivity difference, you may find that it takes longer to heat the glass plate, but it should heat more evenly.