This change is not about this particular SE site, all sites will get a new design. SE has explained some of their reasons for doing this on the main Meta site (Ch-ch-ch-changes: Left nav, responsive design, & themes).
One major reason to change all designs is to unify the core of the designs and make future changes to the entire platform easier. Currently, any UI changes have to take into account a lot of very different designs, and might break on only some of them.
SE is also moving to a responsive web design. The basic idea of that concept is that the website adapts automatically to any screen size. This is a good thing, many websites are moving into this direction, and while it causes some disruption in the meantime, it should improve the layout for many situations. I'm not sure if SE has stated this explicitly, but I assume that doing that for all existing designs separately would have been entirely infeasible and one driving factor for the design unification.
The sidebar on the left is probably the most controversial change, and I suspect that it is mostly because it is very empty right now. There are additional uses for that space planned like custom question lists, but they're not done yet.