Pure silver is not really made up of only silver. It has 99.9% of silver, and a small content composed of other metals such as copper.
Sterling Silver consists of 92.5% silver, and the remaining part consists of mostly copper. This is the reason why Sterling Silver is popularly referred to as 925 Sterling Silver or just 925 Silver.
The reason silver needs to be combined with other metals is that it is very difficult to make great designs with just pure silver, which is very soft and malleable. A bit of hardness has to be introduced, by adding other metals.