I'd never heard of this product, but completely understand the need at the time. Walking into a shop selling computer hardware and software meant most of the shop was unavailable to you because you had to find the one corner where your specific brand of computer was featured. Commodore (64 and VIC20 to the left, Apple II (C and later E) to the right. In the middle a mix of Texas Instrument, Spectrum ZX, and even Tandy in the Radio Shack next door. PC hardware existed too, but was so expensive you'd usually have to go to an "Office supply store" for that.

Having a device like the one featured in the ad makes sense to let Commodore users access Apple hardware and software.