>>725233196
Because you get the latest chapters of everything coming out, they aren't collected in volumes for another 7 months. Most people reading Jump are reading 4-5 series. And even ignoring the shitty quality of Jump's paper (volumes aren't that much better to be honest), you're getting larger pages with more legible details and you get your color pages which are turned greyscale in anything that isn't a kanzenban. Sometimes they'll even use higher quality paper for the color pages, so it's something that's actually worth owning.
The original Jump chapters also contain lots of editor hype text and original promotional art, most obviously the cover, which isn't reproduced in volumes. Not to mention they're the initial versions of the chapters, while volumes and later chapters often contain edits. For Dragon Ball, Gohan says he's 3 years old in his first chapter in Jump, which is changed to 4 years old in the volume and the following chapter, while the original English dub claims he's 5.
