When a bank loans money, it expects to be paid back principal + interest. If there were no inflation, eventually all those interest payments would soak all the money out of the system. This is the reason the Federal Reserve prime rate has such a strong influance on the inflation rate.
If this makes no sense to you, consider that you are a missionary on an island. The islanders have a purely barter based economy, and are suffering as a result. Wanting to help, as well as convert, the islanders, you establish a currancy and become the islands first banker. You loan out 1000 hohos, expecting to be repayed 1100 hohos next year. That will be impossible, because there are only the 1000 hohos you lent out in the first place. In order to get repayed with interest, you have to keep loaning out more hohos so that your previous borrowers can earn them and pay you back. This is inflation.