A wordpress child theme (codex) is a convenient way to make small modifications to an existing theme. More importantly, it persists modifications when the parent theme is updated, without any need for re-applying those modifications. It does mean that BOTH themes need to be installed, while it’s the child theme that is activated.
Most modifications in these child themes are cosmetic, like matching the colour scheme to the respective logo. A few modifications are to meet specific needs of that website or its commissioner.