Some type of room ambience or reflection is important for making things sound more natural, and to give the individual tracks a sense of depth / dimension ... 3-D coordinates in theoretical space to put it another way.
Often, our ears will determine where a sound is coming from based, at least in part, on accoustical information.
Creative use of reverb can help open a track up, close it in, tuck it away, bring it forward even off to the side if used properly. So from a mixing standpoint, it can be an effective tool for "placing" individual tracks ... joining them together more or separating them, etc. It can help out very short transients, like snare hits, to seem louder and more powerful by elongating them, effectively speaking.
Also, from a purely aesthetic standpoint, it can just sound pretty.