With a rebuildable, you get what you want, opposed to what the manufacturers want you to have.
For example, I prefer my RTA builds at 1.4 Ohm, and my RDA at 0.9 Ohm or 0.75 Ohm... cookie-cutter, mass-production builds don't hit what I want, and it seems the sub-ohm tank craze thinks everyone wants 0.2, 0.3, or 0.5 Ohm builds. I've seen 0.8 Ohm cartridge coils, but from there, you get 1.0 or 1.2... I've nothing against the cookie-cutter builds, per-se, but but I want full control sometimes... and my builds seem to last longer, too.
Rebuildables also allow us to configure the cotton (or whatever you're using) the way we like it, not a specified diameter, length, quality, etc.