I don't know that the models usually have other jobs. Some of them, for sure, do. Sofia Kasuli worked at Hooters while she was modeling for TS. But many of the girls who have a larger following aren't able to get a job that pays more than waitressing. A simple google search turns up a ton of information, and then next thing you know, that interview you had evaporates; your coworkers start treating you differently; you get let go for trivial reasons (whether it's legal or not, it still happens). And if you do get that dream job with the great salary and nobody cares about what you did as a teenager online, then the pressure to work and succeed at your job is enormous. You can't just take a random weekend off to go do a ton of photoshoots all the time. And even if you could, would you have the energy to sustain it after working 40-50 hours a week, 52 weeks a year?
So the ideal side job for online model becomes one that is blind to what you do on the internet, flexible enough to allow you the time to go do photoshoots, easy enough to leave you with the energy to do said photoshoots, pays well enough to sustain your livelihood (cover rent, bills, expenses, car, gas, insurance, etc), and anonymous enough that people on the internet won't find you very easily. Not many jobs like that exist.