Sam, Gary is right on the money and this is a well-known trick to increase the depth of the soundstage.

When I use this, I do something just slightly different; I set up a send on each track where I want to use reverb. I put the reverb on a return for the same bus, with the reverb effect set to fully wet.

Then, to place items forward/backward in the soundstage - I will vary the source track dry level, with the track send level - effectively a dry/wet mix setting where I try to keep my original mix volumes the same. If you don't reduce the dry level simultaneously with the track wet level, then the track effectively gets louder. This is key for making piano tracks sound 'real', as if the piano is on a stage, instead of an in-your-face highly panned stereo track. Actually, I should note that panning is key to making this effect happen - reduce the stereo spread of a sound as you move it farther back in the mix, but you can still have it panned. Hopefully the difference is understood.

If you want something to be at the front of the mix, make it mostly dry and ensure the levels are slightly higher than something farther back in the mix.

There are some other tricks you can do to give 'height' to your mix, even though again it's just two speakers.

An old-school trick to make a sound have some movement up/down in the mix is to put a high pass filter on the sound and gradually move the cutoff frequency from low to high frequencies. This can give the impression of moving 'up'.

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