It's not really to prevent you having multiple jobs, it's more to prevent you having multiple jobs at the same time, i.e. the same hours

To stop you taking on projects and then doing that project on another company's time on another company's devices. Lots of people have multiple jobs, the key used to do it at different times of the day.

If a company is telling you you literally cannot have another job at any other time I'm fairly sure they're breaking the law in most parts of the world. You are compensated for the time you work they have no right to mandate your time outside of the time they are compensating you for. If they want you to be available 24 hours a day, then they have to pay you for that.