Another reason why people have done it is if they are playing with bands that have horn players in it. Because a sax is tuned to Eb, it is a lot easier to tune the guitar down than to get the sax player to play things in keys like C#. Someone like Hendrix, who used Eb tuning almost all the time grew up listening to jazz and soul music that had lots of horn parts, usually playing in keys like Eb, so it would make sense for him to always be tuning his guitar to Eb too. It's very often that case that when a guitarist tunes his/her guitar down, it is because their are lots of other instruments in the band.
The othe reason is that a lot of rock players want to keep getting a lowere and lower sound, which is also why things like the seven string guitar become so popular. That semitone down does actually make a difference.