This is a good general conversation starter. It's often used right after engaging in a little small talk, to transition into more serious conversation. One of the purposes of using this word at the beginning is that it announces that you are going to bring up a topic but gives the listener time to prepare before you ask a question.

So, I heard you're leaving us.

So, let's talk about what we need to do to prepare for this conference.

In the example above, saying "So" shows that you want to shift from general greetings to discussing more details about each others' lives.