There is obviously the heritage behind the masks that are worn by the luchadors in Mexico and other countries, so much so that the "character" can be passed on to another wrestler once the original retires- like Tiger Mask.
The mask means alot more in these countries than they do in the US, the US wrestling fans have never really taken to the masked luchador style like the Latin American fans, and the whole style of wrestling is different.
Personally, while I have no problem with masks in wrestling- I think Kane is far better WITH the mask than without, it adds an extra element to his persona, usually I prefer the wrestler without the mask, it allows you to see the facial expression better and the emotions of the wrestler in the match, they can sell much better when you can see they are in pain.
The mask does add a sense of mystery to the character though, like with Kane so they could go with the "burned" storyline, but in the majority of cases I don't really think the mask adds anything, but I know quite a lot of fans disagree.