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In the past, all versions of Processing shared the same sketchbook folder (i.e. location or path). So Processing 1.x and 2.x stored sketches (by default) in the same folder.
3.0 introduces the option to have a new sketchbook folder, so you can have separate sketchbooks for 2.x sketches and 3.x sketches. Enough things have changed with 3.0 that some of your old sketches or Libraries may no longer work properly. By choosing to maintain separate sketchbooks, you can continue to run Processing 2 alongside the new 3.0 release with no problems. Then you can take your time updating 2.x code to work with 3.x.
The downside of having separate sketchbooks is that they won't stay in sync, but the upside is that 2.x sketches and conflicting Libraries, Modes, or Tools won't cause trouble with 3.0.