Why would need S-JIS when UTF8 is around? Just convert it and open it in any serious editing program (even Notepad has UTF8 support).
Because not all of us use UTF8 on tables and such? Because a lot of Japanese games still store their data on S-JIS? Because a lot of
software documentation or game readmes from a few years back used S-JIS? Because some of us prefer to use as few apps to read text files as possible? Because some of us like to use Notepad++ for various romhacking related activities and find this to be a good thing?