Mechanical mods (and a couple high wattage mods) and rebuildable coils are the only way to do sub ohm, so basically you've removed all safety features, good reason to be careful when doing it.
Why though? You'll get a different answer from everybody as to why. Some just do it because they're showboating, some do it for the intense flavor, some do it because they like pushing the limits. Personally, I do it because I'm a heat and throat hit junkie, and I don't have $250 to drop on a DNA 30 ATM.
In any case, rebuildable attys aren't just for sub ohm vaping, but they're the only ones that were built for it.