Most collectors prefer coins in the best condition possible, for obvious reasons.
A coin in mint condition will retain more of the fine detail of the original engraving, ancient coins are easier to read in better states of preservation, and there is generally more pleasure in owning a coin in almost perfect original condition than a worn specimen. These two factors combine, low supply and high demand, so that prices for perfect condition coins are higher than for similar worn coins. In many cases, coins worth hundreds of pounds in mint state might be almost worthless in worn condition. For many years dealers and collectors have used an almost universal scheme for describing coin grades. There are some differences between different national grading sytems, but these are mainly boil down to differences in the descriptions used, rather than any real underlying differences.