The example you bring up is for a single one-off extension. Yeah, for that case it doesn't make a lot of sense.

However, for initial setup of the system (e.g. filling up multiple drawers with baseplates and basic bins, as you will see in many videos online), it would definitely jump start the process a lot, where you'll otherwise spend weeks printing everything. Additionally, if you also go for the fancier baseplates/bins that include the magnets you'll also spend quite a bit of time on assembly and will require external hardware anyways.

I personally didn't think it was a big deal as for me adopting the system incrementally over time worked quite well, but I think there definitely is a niche of people (and possibly businesses) that would like to adopt Gridfinity for its other benefits and appreciate faster initial setup time.