Lets say you're in an area of dim light (20% miss chance). You get in combat with some archers, and so cast Darkness on your weapon/holy symbol. This lowers the light in a 20ft radius, in this case to darkness (50% miss chance). Ignore low-light vision and Darkvision.
Can you still see your enemies (assuming they are > 20ft away)? If so, they would have 50% miss chance against you, but you would only have 20% miss chance attacking them.
Ps. I've read some similar questions, but they were all for other systems or not answered.