We don't want people just stuffing dollars in a mattress. That doesn't help either themselves or anyone else. So we encourage putting dollars/currency into interest bearing vehicles or real assets.

We don't want dollars to sit in hoards. We want them to circulate. If I can be without access to dollars for a while I can loan them to you for some enterprise. Then you pay me back more than I loaned you (and more than inflation we hope) because your enterprise created additional value.

If my excess money circulates I benefit from access to the things that money is used to build in addition to the interest I receive. Since I'm not a medieval landed noble my economic value isn't measured by the productivity of the serfs on my land holdings.