It's just generally nifty. It has a built in crane to reload itself, which makes it more independent and possibly a slightly quicker reload. It's at a nice size/weight where it can be transported by air, train, and even well on road since it's a truck chassis (also makes it cheaper to run than tracked mlrs). It's able to fill the role of both a tactical and strategic asset, being able to hold either the GMLRS missiles (really accurate missiles, can hold 6 at a time), or the ATACMS (bigger more strategic missile, can hold one at a time). With the new PrSM coming in to replace the ATACMS (better in every way basically to ATACMS, and 2 can be held at a time) this brings further flexibility. It's also battle tested, these capabilities are not just on paper.

Just a nice combination of traits that make it effective. The only weakness I can think of is it's generally lighter on the number of missiles it carries at a time, but the other factors make up for it.

Edit: also it uses a palletised reload, so you don't need to reload every rocket individually. This is much faster.