Typically, when you pan a mono track, it just adjusts how much of the signal goes to the left channel and which goes to the right channel. When you pan a stereo track, it adjusts how much of the left channel signal goes to the left channel, and how much of the right channel signal goes to the right channel. So if you record in stereo and pan your track all the way to the left, you don't actually get any of the right-channel signal from the recording.
There are matrix panning functions which allow you to specify how much of each input channel goes to which output channel, but a simple pan doesn't do that.
So, if you record a very lush piano in stereo and then hard-pan it to the left, while you might EXPECT the rightmost part of the piano to be in the center and the leftmost part to be on the left, you just end up hearing only the leftmost part. Which is ass. (What you want to do instead is use a matrix pan to put the left channel to [1,0] and the right channel to [0.5,0.5], for example.)