Ballpoints and fountain pen deliver ink to the paper very differently! You can't really fill a fountain pen with thick ballpoint ink because it would jam the feed very quickly. Likewise, if you were to fill a ballpoint with fountain pen ink it would leak as soon as you touch its tip onto the paper.
However, thanks to fountain pen ink being thinner, you don't have to apply nearly as much pressure when writing. Like many things in engineering, it's all about tradeoffs!
That way, you avoid the wastage of throwing out empty cartridges, which are effectively unrecyclable because of the leftover ink inside and also because they don't carry material markings. They also hold considerably more than a converter, and not all Pilot pens (such as my Decimo) can even take a converter in the first place; worse, even if your pen can take a converter, refilling one at work or similar is a messy, time-consuming pain in the neck, where cartridges are easy to carry and quick to swap.
Whether you use a cartridge or a converter, it's also worth noting that inks formulated for Western pens often have trouble flowing in the narrower feeds and finer nibs of Japanese pens; I've had considerable trouble with Herbin and Monteverde inks in my Pilots, while they flow perfectly well in Western pens with broader nibs. Something to keep in mind to avoid the frustration of constant hard starts and other flow problems. Pilot's Iroshizuku line is well matched to their pens, and offers an extensive range of colors; for a Metropolitan, especially one with a nib narrower than M, I'd recommend starting there.
Used to use the pilot disposable fountains myself, they're pretty good.
Why is it that a fountain pen needs to be filled in a matter of weeks and a cheap Bic Cristal never runs out of ink?
I have an inexpensive Pilot Metropolitan that I do like writing with, but the amount of ink I put through it seems crazy for how much I write.