Redirects in Wikidata are basically the same as redirects on normal wiki pages. However, they have a slightly different meaning and intention.
The main reason why we need redirects is because we want to provide stable identifiers.
The merging of two items has been a common place task since Wikidata gain momentum as often two different people or bots create items representing the same topic or concept, thus the data from one needs to be moved into the other and the empty item removed.
The problem with this is that removing the empty item of course means that one of the two identifiers no longer points to the topic or concept that is represents. This is therefore not a stable identifier..