Most of those BL stuffs are written by women. So yah, I am kinda skeptical.
For the same reasons they make similar products for vajaja. To generate the lubrification effects, you have to stimulate that part : that's the natural process which is self-sufficient. In some cases, it might be tricky to generate such ideal "moisturizing" from stimuli. Thus the lube that makes you skip the natural process. The lube is not necessary, it is efficient.
It's marketing too. Some come with nice flavor (?), scent, sensation-triggers and even aphrodisiac. So ways to set the mood.
In the same way, the vajaja is self-cleaning, yet there some weird PH-control soaps to clean it... Even condoms come with some gel, the lube is extra. Most aren't dry.
In short, the Black hole is self-lubricating.
Just be sure to be hydrated before and after.
Omg... the. anus. is. not. self-lubricating.
Honestly one of my biggest pet peeves when it comes to BL (╯°Д °)╯╧╧