Some apps install things in other places. /Users/Shared/ for example, or /Library/Sounds/ etc. iLife for example, installs imovie and idvd themes and garageband loops in /Library. The apps will still run on another machine (probably) but won't have all their stuff. Some apps require their extras to be where they should be. A few keep a copy inside their application and will reinstall them if they're missing. (appleworks and ms office are good examples, they install any fonts they need when launched)
