I have been noticing lately that every app. I download, has a message of notification advising the file is a particular sort of file - APP2SD.
The message asks whether to to install it in the phone or on the card.
I don't recall that this message was previously delivered with earlier apps I downloaded.
It seems to me the phone would be better & faster if it had less clutter, and I have opted to move all apps. to the card.
There is always the option of moving them back to the phone. And I guess you might want to do that temporarily if you were changing the card for another. Then move the over to the new card.
Perhaps that was until recently I went to the market directly with the phone, instead of downloading them from my PC.