Having a professional mix your tracks can drastically improve the quality of the end product, whilst also requiring significantly less time than it would take the average hobbyist to achieve the same results.

Mixing is the production that happens after recording before the tracks are mixed down into one file for mastering. Firstly, the best takes for each instrument are merged together to create a perfect performance. Then timing of drums and all other instruments can be easily tweaked to make the song fit well together, whilst the vocals are tuned by hand.
Next instruments are treated for their dynamics and tone, using EQ and compression to give clarity to the mix.
Effects are added according to taste, style and genre.

Additional digital instruments (particularly keyboards and pianos) can also be added. The quality of digital instruments has improved significantly in recent years, and are now very much a viable option in professional recordings. Drum replacement (replacing the original recorded hits with those of better quality) is another useful tool, which can also be used to add to the natural sound to get a more electronic feel, whilst accurately matching the timing of each hit.

Tracks recorded elsewhere can be mixed here.