On expiration day, a market maker with a cost structure that might be $0.20 to $0.25 all in, can sell expiring options for $1.00 and make more than $0.75/options. at EOD, the options expire and do not appear on their risk or margin requirement.
If I own 100 options that are about to expire, I could sell to close: 100 (* 100) * 0.01 = 100$ If commission is less than 100$, I will end up ahead.. Some benefits: 1) Possible to make a little extra money (as opposed to letting them expire worthless) 2) Selling increases volume, and the higher volume may kick you into a lower commission bracket 3) Can settle your books early (for those who are strict on bookkeeping) 4) Can effect your risk management (an open position, even if 0.01, can count as a risk that needs to be managed. But closing it out will of course mean there is no risk there anymore). Regarding #1.. the long term maths would be: 100$ / position. If you trade 10 positions per week that would be 1000$/week. If you do this every week for a year, that would be 50,000/year. So it can add up. Many people will calculate earnings on an annual basis, so this would factor in significantly. Might even be able to cover the cost of a Bloomberg terminal
Often, I bought back my short option positions when they were <$0.05 so someone was willing to sell them. They thought it was easy money.