When to use 了 and 的?
他（是）现在才来的 is a 是的 construction for emphasis. In this case, 是 is omitted. He has arrived just now, in this very moment!

Another example: 我是来看病的: I have come to have a health check (rather than just socialize).

他来了 just indicates he has arrived, without emphasis. 了 completes the statement.