you use it for music, pics, vids and things of that nature..... not applications..... it isn't like a removable hard drive, more of just a storage unit!!
I mostly use my memory card to store pictures that i have taken with my phone or pics that people have sent me..I have an ipod so i don't really use my bb as a media player..However, since the Tour has a much higher res then my curve, i might put some funny videos or clips on it.. not sure...
Kind of sucks you can't run apps from a card but with the business perspective I can see the security issues. It was a nice feature on low memory WM phones.