이클립스 프로젝트 클린을 하는 이유

Clean is useful if some external tool modifies your output folder. For example, you are using Eclipse, but occasinally compile via command line compiler into the same folder. In this case Eclipse may fail to do incremental build and display errors in code when they are actually absent.

Another case is working around some bug in Eclipse compiler itself. In very rare cases and in specific Eclipse versions/updates some classes which are necessary to be recompiled after specific code changes might be overlooked by compiler. If you were (un)happy enough to encounter such case, clean will help you.

나중에 자세히 찾아볼 예정

출처 : http://stackoverflow.com/questions/30031952/why-use-project-clean-in-eclipse