If you've switched to the new Google Maps desktop interface, you can quickly go back to the classic interface by clicking the "Help" icon at the bottom of the page and picking "Return to classic Google Maps". You'll see a message that asks you if you want to permanently go back to the old interface or temporarily switch.
Another option is to bookmark this URL: Classic Google Maps (https://www.google.com/maps?output=class). It will always send you to the standard Maps interface and you won't have to open the new interface first.
Why would you switch to the old interface? Maybe you need to use the more advanced directions that let you add more than 2 destinations, maybe you like the terrain/photos/webcam layers or Maps Labs features like the distance measurement tools or the LatLng tooltip, maybe you miss the Pegman icon you could drag and drop to switch to Street View or you want to customize the printing format for directions.
You've probably noticed that the new maps style (with new colors, icons, labels, tooltips) is now also available in the classic Google Maps. If you hide the sidebar, you can make it look more like the new Google Maps.