Once you have a registered trademark, you’ll likely get solicitations to help “police” your mark. They’ll offer search services and to remind you of upcoming deadlines. (Talk to your attorney before you sign up for one of these services, it’s possible that your lawyers are already doing all this for you.)
It’s important to keep an eye on other trademark filings, and other people potentially infringing your mark. One big reason is that the Trademark Office isn’t perfect, and sometimes they let confusingly similar marks, and occasionally nearly identical marks, through the approval process.
If you or someone on your side isn’t keeping an eye on new filings, your mark could be diluted by the registration of a similar mark. The more similar marks there are in a given space, the easier it is for someone to register a similar mark in the future.