Yes! To let the user test the registered username password combination.
Additionally we train the user how to sign in to our site. The next time the user visit our site, she/he know where to find sign in, and how it works.
When the user registers, she/he does it on free will to accomplish a task or access content. The motivation is high at that moment which make that moment perfect for training. There is a risk, however, that the user might feel that the process takes too long. The risk of a too long process stands in relation to the user needs and motivation. The less need and motivation the higher risk of an unsatisfactory user experience.