I include people in my photographs whenever possible for three reasons: 1) I like people, 2) most people like me, and 3) people can give scale to a photograph.
For example, these stainless fermentation tanks at Changyu Winery in Yantai, China are big. "But how big?" you ask. Check out my two Changyu marketing buddies Edward Dong and Michael Lau (their Englishized names) at the bottom. Yea, my ant-sized friends silently scream the exact size of those monster tanks.
So the rule here: Use humans as yardsticks.
Thank you Edward and Michael.