Registration: necessary for instituting infringement suits, and for obtaining statutory damages and attorneys fees

Term: copyrights have a long duration, e.g., life of author plus 70 years.

Assignment: requires a written agreement.

Ownership: paying for a material object (such as a painting, book, CD, etc.) does not necessarily mean the copyright to the work embodied by that object has been purchased.  That is, very generally, even if under contract to create a work for your company, a written assignment of copyright typically is needed from any independent contractor, and from any employee working outside the scope of their employment, if the copyright is to be transferred from the creator to your company.