You only understand how important a second camera is on the day you desperately need one. In 2018, on a ship with my family and half an hour of free time, I vlogged about why every serious creator needs a backup body.
My backup: the Sony a5100
If you look closely at the reflection in my glasses, you'll see I'm not filming on my a6500 today, but on my Sony a5100 with the original 16-50mm lens. It has no microphone input, so I record audio separately on a Tascam DR-05 and sync it later. It's easy, and the results are great. The best part: the lenses are interchangeable between the two cameras. I paid just 299 for the body with a lens.
Accidents happen
If you're working for a client who's paying and expecting a job done, at a wedding, a photoshoot or a music video, you can't rely on a single camera. Imagine it falls and won't switch on, and you're 50 km from home. What do you tell the client, especially at a special event? You need a backup. The moment you want to work like a pro, you need a second body.
More angles, fewer mistakes
Multiple cameras also mean multiple points of view. When I'm out shooting, I keep the a6500 in my hand or on the gimbal, and put the small one on a tripod for a wide shot of the whole scene. Sometimes that little camera saves my day, when I miss focus or the audio on the other mic goes wrong. It also lets me film myself at work for short behind-the-scenes vlogs.
A second operator would be even better, but I don't have an assistant. So the camera on the tripod is my assistant. You do what you have to do.
Bottom line
Even though this was still my hobby, I felt the need for a second body. The a5100 is cheap, a really good camera, and even a perfect first camera if you're starting to vlog. (Sony isn't paying me to say this!)