You own the copyright to your photographs from the moment you take them, and registration of a copyright claim is not a condition of copyright protection. However, in United States courts, and particularly for works of U.S. origin, registration of your copyright brings advantages, including two major benefits:
- For works of U.S. origin, you may not file a lawsuit for infringement of your copyright until you have registered it. The only exception is that if you submitted a complete registration application that was rejected for some reason, then you may file a lawsuit for infringement if you provide notice of your suit to the Copyright Office. The Copyright Office may then join the suit on the issue of registrability. 
- Generally speaking, you may not recover attorney’s fees or statutory damages for infringement that took place before the effective date of registration. Statutory damages may be preferable when actual damages from the infringement are difficult to prove, or when a judge or jury wishes to punish the infringer. The effective date of registration is the date you filed a complete application, not when the application was processed, which may be months later. For published works, there is a grace period for registration for three months after first publication. 
In addition to these benefits, copyright registration also serves to create a public record of your copyright claim, and registration within five years of publication serves as prima facie evidence that the copyright is valid.
So in order to sue an infringer, you need to register your copyright, and you are in a much better position if you registered your copyright before the infringement took place. In short, registering your copyright provides stronger protection against infringement. For this reason, many photographers make copyright registration part of their normal workflow. It is worth noting that copyright registration is a unique feature of United States law. Other countries do not have such an elaborate system of registering copyright.