I am a research scientist, and as part of my role I am expected to perform some form of public outreach and engagement. I enjoy outreach activities and giving public lectures.
I have a suspicion that free public lectures are usually poorly attended. I suspect that the attitude is one of "Well, it's free and you get what you pay for". Certainly the quality of artistic performances that I've seen is strongly correlated with entrance fee.
While it is a reasonable expectation that my Faculty would pay for the costs of lecture room hire, advertising, possibly even tea and biscuits, I wonder if there is an advantage of charging a nominal fee to attend, to overcome the low-quality perception of a free lecture.