A child theme is a small folder that sits on top of the real theme and overrides pieces of it. That is the whole concept.
When you need one
Only when you are editing theme files. If everything you are changing lives in the Customizer or the page builder, a child theme adds a folder you have to maintain for no benefit.
When you do not
Custom CSS, colours, fonts, layout widths and header behaviour are all settings. Changing them does not survive-or-die with updates.
Setting one up
Every Novex theme ships one. Activate it, and any file you copy into it takes precedence over the parent — and updates to the parent stop overwriting your work.