This DIN-Rail opto can be handy when you need to convert signals from 24VDC to for example 5V TTL. This often occurs when working with CNC interfaces like USBCNC/Mach3 etc, since most of those pcbs are 5V TLL. Then most sensors work on 24VDC. Also it protects the vulnerable 5V TTL circuit, since any short circuit in the 24VDC line can never reach the 5V TTL side. Keep in mind you will always need two parts, the Din Rail input socket, and the the swappable Relais or SSR. Different types of SSRs and Relais are available so you can convert to different voltages.