Why and when you need a step-down ring
The opposite of step-up rings are step-down rings, which essentially do the reverse: they enable you to adapt filter and lens accessories with smaller thread sizes to lenses with larger thread sizes. For example, they allow you to attach a 52mm filter to a lens with a 72mm thread size.
Step-down rings are not as widely used as step-up rings because, in most cases, the smaller filter sizes result in vignetting. The exception to this rule is when using lenses and filters designed for larger-format cameras (i.e. full-frame) on smaller-format cameras (i.e. APS-C and Micro Four Thirds). In these cases, the smaller image field will often negate any vignetting or image clipping.
For further reading about the filters to which you can apply step-up or step-down rings, click on the following links:
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