I assume it's an effect that plays when one taps the back arrow in the bar. I'm using Opera on my phone, no animation there, so I can only guess.

The are page transitions. Considering that Google controls both the source and destination page I'm sure they can make one without stealing space on screen. That annoys the user more than the owner of the content.

And if the user has a browser that doesn't support page transitions, it's only a waste of screen space.