The grammar is a bit more hand-holdy and ordered in a way that’s a lot easier to digest. They have SRS for grammar, as well as specific grammar drills, games, base vocab SRS and achievements and yearly wrap ups. Basically you are getting multiple language features for the price of a single app. Things like WaniKani are good, but you pay for a certain amount that MaruMori covers and that and then some. So value wise it already beats out WK, and tons of people love WK for it’s SRS. It’s basically best value where you get most other app features in a centralized app. It’s not perfect, but it just has a lot of value baked in that’s hard to find elsewhere. I think Renshu is the nearest thing, and even then the grammar lessons on that are somewhat lacking by comparison.

I use paid services more often than not, I’d love to learn without having to pay, but if a service is good I’ll use it. The only language I can even think of to learn for free would be Lojban, and that’s because it’s so niche there’s literally no demand for it, so there’s no products for it. Maybe after a next language cycle when I feel I’m just going to maintain my already learnt languages, I’ll cruise on into Lojbanistan.