Maybe you need one of your staff to stay later than usual. Perhaps you’d like a colleague to fill in for you at a meeting, or swap days off with someone. You’ll have a higher possibility of winning if you ask individuals, not groups for help.
For the same reason (so they don’t know who you’re asking) bcc if you’re going to send an email asking for a favour to a group of people.
The idea is they think someone else will do it if you ask more than one person. Better to target (or look like you’re targeting). So says some research from Influence at work. They have moved the link, and the research is outlined in The Small Big, which I’m just reading now. It’s partly the inverse of the principle of social proof (someone else will do it).
You’ll have to take my word for it!