A noun followed by も is another way to establish the topic of a clause. In this case, the new topic is introduced in addition to the previous one, not instead of it.
This is Mr. Brown.
There are a few fundamental reasons to communicate that are common to all languages, such as giving and asking for information, exclamations, and asking or telling somebody to do something. Giving information is the most important of these. Sentences that give information are built in a similar way in all languages: they say what is being talked about (the topic), and they give information about it. (the predicate).
The English sentences, “I’m cold”, “The cat sat on the mat” and “This is Mr Brown” all all information-giving sentences. In these examples the topics are “I”, “The cat”, and “This”, and the predicates are “am cold”, “sat on the mat”, and “is Mr Brown”.
In classical English syntax every sentence that gives information has to have a subject and a verb. These are roughly equivalent the topic and predicate as outlined above, which means that in English, you end up explicitly stating the topic in every sentence. However, in Japanese, a predicate is enough to form a complete sentence, and there is no syntactic necessity to state the topic, so if it is obvious you are talking about Mr. Brown, the sentence 「ブラウンさんです。」is all you need.