When you used the Home button when you're finished with an app, the app keeps running (all Home does is run the launcher). If the app has anything stored in cache (and almost all apps make a lot of use of cache), when you turn the phone off, restart it, or Android needs the space and kills it, cache isn't cleaned out.
If you use the Back button when you're finished with an app, it cleans out its cache, then stops. Still one button, but doing it the right way. And cache stays pretty clean. (I usually don't have more than 25MB or 50MB in cache, which is nothing, even on a 16GB phone [it''s less than 1% of total storage].)