Some apps that were not developed correctly or that run constantly can cause you to lose your application free memory. You can check it by going into Options-->Memory.
To add, think of running several applications (iTunes, Windows Media, multiple streaming video/music in your web browser, Excel documents, .pdf files) all at the same time on your laptop. It'll cripple your computer and cause it to run at a snails place.
Same with the Blackberry, in both instances you check memory.
