Learn Russian: Этот человек является лидером, а это важный фактор. - This person is a leader, and this is an important factor.

It directly follows the verb является. Formal verbs of equivalence like являться require the noun or adjective that follows them to take the Instrumental case. Here, the masculine noun лидер takes the -ом ending.

Since является just translates to 'is', why not drop the verb and say Этот человек — лидер?

You absolutely can, and in everyday casual speech, you usually would! However, являться is used in formal, academic, or professional contexts. It adds a tone closer to 'serves as' or 'constitutes'. You will see it a lot in news articles and official presentations.

Why do we use Этот at the beginning of the sentence but это in the second half?

At the beginning, Этот is a pointing word (demonstrative adjective) modifying a specific noun, so it has to match the masculine человек. In the second half, это acts as a standalone pronoun meaning 'this is'. When used to introduce a statement, it always stays in the neuter это form, even though the noun it introduces (фактор) is masculine.

Why use the conjunction а instead of и for 'and' here?