Aesthetics, mostly. Renge is pretty. There seems to be some speculation about whether their presence indicates a faster stone, but it's as far as I can tell it's all anecdotal. Suita are generally on the more expensive side for jnats, and renge only adds to the price. The prices of jnats in general and suita specifically have increased quite a lot in the last few years and the vendors you mention have followed that trend. That said, people tend to gravitate toward suita (and often ohira suita specifically) because as a general rule, they tend to be a good combination of a useful grit, relatively good cutting power and speed, and attractive aesthetics all in a fairly easy to find package.
By this I mean to say:
Are there better deals? Probably. For instance, a fine-grained tomae (or just a plain-looking suita) might ostensibly accomplish the same thing as a renge suita at a lower price.
Can you get your knife to the same level of sharp with a different, possibly cheaper jnat? Almost certainly.
Is a suita with renge worth it? That all depends on you.
It helps to keep the following in mind when shopping for natural stones: Anything to do with jnats- mine, vendor, color, appearance, size, pictures,video etc- is simply information that you as an end-user can use to help you make a reasonable, educated deduction about the performance of a stone. But you never truly know until put a blade (or several) to it. Natural stones aren't like knives where you can quantify things such as steel, heat treat, distal taper, grind, etc or like synthetics where you can measure grit particles, wear speeds, etc. Jnats decidedly lack in precise, measurable qualities. You can never be 100% certain that what you're buying will meet your expectations or requirements. There is always an element of risk that no amount of questions, research, or deductions can eliminate.
