What an interesting phenomenon that's not quite modern and not quite classic. Your friend who cuts hair or takes grad photos does so with a little more seriousness than a hobby, but not quite the institutional legitimacy of a job. Something like the young adult form of babysitting, maybe.
I say "not quite classic" because it's certainly a phenomenon that's on the rise; in previous generations of course holding part-time jobs outside of school or outside of your primary profession was by no means unusual, but in the past it hasn't been conducted on an informal basis, under the table, amongst friends. Nowadays the side hustle has become a kind of friendly black market communal economy, with colleagues helping each other out for affordable services like (to reiterate the two biggest ones I've encountered) cutting hair, taking professional photos, and yes, editing (I'm lucky enough to have turned my former side hustle into a career, but that's the exception to the rule). A quid pro quo, with Venmo operating as the central bank. For some it fulfills only the necessity to have a little spending cash, but for others it's a necessary practice in order to end each month in the black.
But I say "not quite modern" as well, because we have seen this before if we go back far enough. If you return to a time when a substantial portion of the population still practiced agriculture (not necessary pre-industrial, but we'll say pre-suburb), this was the law of the land for those outside the city. If you needed a well dug or a home improvement made, if you needed something smithed, et cetera, you sought out someone nearby who had the basic skills and equipment to do this even if their trade was something irrelevant. For these services you paid either in cash or in kind, and of course never taxes. It was an economy that existed in the absence of affordable professionals for these services. Just as today not everyone can pay a professional editor but everyone needs something edited, so too back then could not everyone afford to pay a smith to make nails, but everyone needed nails.
So what does this say about the Millennials, who've revived this not-quite-classic economy? Well, for one, I'd say it rejects the notion of entitlement. These side hustles often stem from things the practitioners would do as a career if it were lucrative enough, often were the result of "chasing your dreams" before deciding to "study something practical." Entitlement would suggest that the inability to make a career out of the side hustle would cause complaints, but instead the existence of a career, usually in a different field entirely and one closer to what the practitioner studied or studies, shows the willingness to work and, as critics are wont to say, "be practical."
Second, it demonstrates a divide between those who can afford to pay for professional services and those who can't. The beauty of capitalism of course being that no money can be made off of things people don't need or want, and therefore the fact that there is a black market for affordable haircuts and grad photos means that the practitioners have seen an opportunity and exploited it. By that same token, the fact that the practitioners need that side hustle for extra spending cash, or that they can't make a profession out of the side hustle, shows that wages are too low for a career alone.
If ever there was a definitive measure of income inequality, the rise of the side-hustle was it: It existed in the pre-industrial and premodern, it existed during the great depression, it went away during the prosperity of the second half of the 20th century, and it's returned now.
But that's not to say any practitioner or consumer of the side hustle should be condemned. In fact, the opposite. Those who practice the side hustle and those who consume it are those who clearly need it, so without sounding too self-serving, do the charitable thing and support your local side hustle today.