Copernicus was one of a number of people who were trying to solve the problem of why astrological charts kept coming out wrong.
Basically, this was because the old system of predicting where planets were kept breaking down, because it had the Earth in the middle. Copernicus realised that if you put the sun in the middle, then you could predict where, say, Venus would be with much greater accuracy. And that's useful, because it meant you could publish a much better set of astrological charts - one that didn't keep having planets in the wrong positions.
A good period summary of Copernicus' theory is in an appendix to Leonard Digges' 'General Prognostication', which is available as a Da Capo press reprint. The same book also has a good explanation of the older, Earth-centred theory.