Its not necessarily about disabling javascript... its that js is often a poor way of detecting browsers. Remember how the User-agent header was supposed to identify the browser? All the browser's put arbitrary crap there so they will be served this or that version of html. Now browser's are all trying to look like each other in js land for the same reason -- so that apps work right. Sometimes these css subtleties are all you end up with.
The reason you would use this is that you can optimize your application in all sorts of arbitrary ways. maybe string concatenation is faster a certain way in safari, great; lets just make sure its safari before we do that optimization.
Sometimes I follow links to poorly implemented or managed sites, some of which pass along unfiltered content. I disable all executable content by default to ensure those places don't inadvertently screw up my browser state. For me executing javascript as a privilege I'm only willing to extend to sites that have earned my trust.
