By using Categories and Keywords together, you can group and better organize your records. When doing so, think of the Category as the main group, and Keywords as sub-groups.
For example, you may have the Category Lead or Client and use Keywords for each type of product that they are interested in. This way you're able to filter all clients that are interested or have bought a specific product, and you tell them about an upcoming promotion on this product.
Another example would be in the photography industry, using the Category Prospect and the Keywords wedding, engagement photos, portrait, etc. for any of your services that they might require.
This concept spreads to other objects such as Projects and Opportunities as well. A consultant may have a client project that is the Category is On-site or Phone Call for the type of service offered, and the Keywords are Internet set up or software installation to express the specific applications of their service.