If you have a module in your eurorack that needs a 5v supply, you're going to need one of these. An example is the Sputnik 6 channel mixer, and some midi to cv interfaces. Product is simplicity itself - just plug it in to a spare slot on the rail you want 5v on, and the job's done. Probably easy enough to make yourself, but at this price, why bother. Just make sure you plug it in the right way round, as there is some margin for error here - red stripe goes at the bottom!