Cassettes come in a number of different speeds and ratios. When the time comes to replace your cassette, upgrading to one of a better quality can result in smoother, and quicker shifting, ensuring that when you change gear the drivetrain is working to its full potential. More expensive cassettes often last longer due to special coatings on them, designed to prevent premature rusting. A change in ratio can also make it easy to get up the hills if you are struggling, or make the bike more race-focussed for the more serious cyclist.
Need help choosing a cassette? Read our guide on Choosing the Correct Cassette