Why?

Mix flour and water, let it sit for 5 days, adding a little bit of flour and water daily to feed it.

You don't need to purchase a starter.

Mostly to see what it's like. The businesses selling starters focus on historically interesting, important, or notoriously good starters (like the "San Francisco sourdough", or the oldest known starter in the world, salvaged from an Egyptian tomb). I've done my own wild starter, and my daughter has also done wild starters (in her case, she made a rice bran starter for making Japanese nukazuke pickles).

So mostly, it's the academic interest.

I like the idea of incorporating locally available microbes but forgive my ignorance: is it safe?

Oh, it's perfectly safe. You can tell by the smell... a healthy sourdough culture smells healthy, a rotten culture smells rotten. The yeast/bacteria system is extremely tough and resistant to other microbes and molds.

Whenever you feed your sourdough starter, or add locally bought flour, or anything along those lines you are introducing locally available microbes.

A sourdough starter from SF won't be a SF sourdough for long just because of the microbes in the air/flour as you continue to feed it.

It becomes it's own living and breathing organism that you have to take care of or it dies.

I did it for years and my family's still here :-)

Survivor bias!