Generally, the person who puts together your to-go order does an amount of service for you in order for you to be able to bring your food home. They generally are checking to make sure everything is correct, organized, and prepared at the time they explained to you. There are many places where there isn't a to-go specialist, so the work is done by a bartender. It's plausible in that scenario that the bartender is taking time away from their work to get the to-go order together and in order with everything necessary.
Just saying that there are definitely situations in which tipping the person who did work for you is appropriate.