A serial comma (or Oxford comma) is an optional comma used before the last item in a list. For example, “bread, butter, and tax evasion” uses a serial comma, whereas “bread, butter and tax evasion” does not.
Do you use it? Why or why not?
I always use it. I don’t perceive any less of a pause between the last two items in a list than between any others, so it feels natural to put a comma there as well. Tbh, I’m so used to it that I usually have to do a double-take when it’s not there (since it looks like a grammar error to me at first).
I use it because it feels like otherwise you are lumping two things together. In your example it would be butter and tax.
Another example could be talking about people:
Without the comma I feel like that implies Ian and Susan are in some kind of relationship.
With food:
That seems to me like someone had a ham and cheese sandwich versus ham and cheese separately.