After you've had your trademark application filed, it's absolutely necessary that you monitor your trademark in some way. The USPTO does conduct a search of their own files so if someone does file for a Federal trademark after your registration it typically is rejected. However, relying on the USPTO to protect your trademark is a mistake for several reasons.
First and foremost, the USPTO is going to operate by their guidelines, which are strict to an extent. However, they may not be strict enough for your own liking. Let's look at a real life example:
Norcross Safety Products has a registered Federal trademark for Ladybug for "garden clogs and garden boots," which is in International Class 25, the clothing class
TSP Fashion has a pending Federal trademark for Lady B. for clothing of all types in International Class 25 and specifically mentioned "shoes, sandals and slippers"
On the surface they look different except in looking at the record for Lady B. you can see that there is also an inclusion of a design of a ladybug. Again, the names are different but the message of the name is similar as is the industry, namely clothing. This is not to say that the USPTO is wrong in not refusing Lady B's filing but it does prove my point.
Let's just assume that Norcross would take issue with this mark. Unless they're monitoring their mark in some way, Norcross would not have a clue that Lady B. is close to becoming registered.
Second, it can take the USPTO almost 6 months to conduct a search of their records. This means that a competitor can be using the same or similar name that whole time, which means you're likely losing customers and if so, you're definitely losing money.
Third, the USPTO only searches their records, which consist of the Federal trademarks. Yes, that's a large number of records however State trademarks are going to be relevant to your business as well.
Let's say your business primarily does business on the West Coast but you have plans to expand to the East Coast. A Federal trademark does give you the right to the entire country, however, if someone has a New York trademark you'll have new hurdles to deal with in expanding that can be time consuming and costly.
Those are three important reasons why monitoring your registered trademark is a necessary part of your business plan. There are a couple of different ways you can go about monitoring your mark - 1) hire a trademark monitoring service OR 2) have comprehensive trademark research conducted every 2-3 years.