The answer to your question is yes it's called sub-contracting. Businesses do this because they may require skills for a project that they do not have available to them in-house and/or because it is more cost effective to do so.
Freelancers are no different in this respect. As for informing the client, it may be necessary depending on the project but you should not think of this as being a negative.
All the client cares about is that you deliver the end result to their satisfaction.
