Many macros have interesting compile-time side-effects that would normally be hard or impossible to replicate without integrating with their internals.
evaled-when allows you to trivially and portably extract and replicate these compile-time side-effects, which can be useful if you're trying to make a better version of defclass (as indeed I am), for instance.
One should normally import a single symbol from this package, evaled-when. Any other symbols from this package should normally be explicitly qualified, such as evaled-when:parse-situations. Or you could import those you use a lot. Don't (:use)!
(&rest situations) &body body &environment env => nil