It's "needed" if GM wants to appear competitive with the other manufacturers that are going with 6-speeds.

Technically you could gear a 5-speed to give you good acceleration in 1-4 and then good cruising fuel economy in 5th, but most drivers wouldn't like the tall "jump" up to 5th. And most people are too brain dead to downshift from top gear when they need acceleration (which is why they give us so many cars spinning at ridiculous RPM cruising in top gear).

The extra 6th makes it more consumer friendly.

I just hope GM doesn't go the way Ford is with the new Fiesta: the six-speed automatic is geared to get better economy than the manual.