We’ve learned that a predicate can be made with [noun] + です, and we’ve also learned that if the topic is obvious, there’s no need to say it.
This is Mr. Brown.
We’re still talking about Mr. Brown, and now we can carry on using the [Noun]です structure to ay more about him.
He is a student.
However, the listener didn’t know we were talking about him, we’d have to establish him as the topic before we could give any information. This is done using a particle は, which shows that the noun phrase ブラウンさん is the topic.
Mr Brown is a student.
This is very different to