I've looked into this before not understanding why there were two ways of doing the same thing. Dagger1 advocated @Qualifier afaik and then Dagger2 started to provided both. It maybe a nod to the many other java (not Android) systems that could be wanting to use earlier ideas proposed in JSR (specifically Spring?) in order to adopt Dagger2. I read this:
http://www.adam-bien.com/roller/abien/entry/what_is_the_relation_between