If another company files a federal registration before you do, you will be locked in to only using your trademark in your current geographic area. Therefore, your business will not be able to expand.
If another company starts using your name it could hurt your reputation. For example, if the CEO of another company is caught embezzling funds, the public at large may assume that your company was involved if that other company is using your name to identify themselves.
A federal registration may function as a pseudo shield against claims of trademark infringement by third parties. The USPTO vets out improper trademarks and trademarks that are confusingly similar to prior registered trademarks. Furthermore, after five years of registration the trademark receives a presumption of incontestability. Therefore, after a trademark is federally registered it is much more difficult for third parties to claim that such a trademark is infringing their own.