Here's my take on panning(and I admit that I am still working on this myself so maybe I'm only barely ahead of you
). You won't get "surround" feel out of stereo, but some tricks are worth trying(giving a stereo mix "depth" or a "3D feel" is what separates the good from the great). Yes, there are widener plugins and I use Waves S1 and Vitamin a fair amount, BUT, as much as you might be tempted to slap that on the master, don't do it. Because(I know you just asked why
) they work by messing with the phase, so using it on the master has the potential to do as much damage as it might give the illusion of sounding bigger/wider/better/etc. When I use wideners, I tend to stick it on a reverb AUX track or some VI that is more of a texture than an "instrument" sound. Getting "depth" is tricky and some are better than others(I consider myself closer to the "other" category
). My best advice there is; dryer signals will seem more forward and wetter signals will seem more distant, so start with that concept and see what you can make happen.
Last 2 cents for those that are newer to this stuff, just because you CAN do something, doesn't mean you should. Example; its easy to stack tons of tracks when you have (essentially) unlimited tracks in the digital world. I had a client send me a session to mix and it had 8 guitar parts, and each part was done with 4-6 tracks(different cabs, different mics, close and far....) so it was around 30 tracks of guitar
I pared it down to 1-2 tracks for any given part and ended up with a mix that blew the client away(sometimes less really IS more, and this can be a valuable lesson
).