Yeah, it’s kind of a fringe thing, but I myself own a true mono cartridge in addition to my stereo cartridges. Why? Because I own and enjoy a ton of classical, jazz, and other pre-stereo, mono era–only records.
A stereo LP's grooves are cut with both horizontal and vertical modulations to produce a signal, while mono LPs contain only horizontal modulations for the signal. However, LPs generally attract dust n dirt into their grooves vertically, and that's where a mono cartridge comes in handy: Since a truly mono cartridge is designed to only pick up the horizontal modulations, using a mono cartridge on a mono record can be much, much quieter in terms of background noise than using a stereo cartridge on same. This is because the mono cart’s generator coil assemblies are 90 degrees/parallel and perpendicular to the record surface instead of 45 degrees/in an X-shaped cross like with a stereo cartridge. Note that a real mono cartridge is different than a cartridge that is just internally strapped for mono; there are an awful lot of cartridges out there whose generators aren’t really “mono-oriented” and their construction is no different than their stereo counterparts: Their two channels are just simply combined before the output pins.
I buy a lot of old, used mono records on the cheap whose grooves have accumulated a lot of gunk, and it seems like no matter how much I clean some of ‘em, some of that gunk is just too deep to get out; having a cartridge that won’t turn a lot of that gunk into noise is a huge blessing when we’re talking about records of solo piano or violin sonatas with quiet passages throughout. And since the mono cartridge I have requires the same alignment/set up parameters as my stereo cartridges, all I have to do is swap one cartridge/headshell combo for another whenever I go from a stereo to a mono record.
So hope this helps explain the mono thing a bit--it's a noise thing (or rather, reduction of). If you enjoy a lot of pre-stereo era records, a mono cart is definitely not required, but it might just make your music a little more enjoyable.