This is a good way to signal a new "part" of a conversation.
For example, when you're starting a meeting, you can start by welcoming everyone and sharing general information. Then, when you're ready to talk about the main point of the meeting, you can use "So":
So, let's talk about our budget.
You can use "So" to switch to a more serious topic when talking with friends or family:
So, how are things going between you and Kelli?
You can also use "So" to transition between describing a problem and talking about the solution:
...and now we're losing over $200,000 per month. So, obviously, we need to talk about cutting costs.