In case of Vivaldi, it's features like vertical tabs, and extreme customizability for the built-in stuff (for tabs alone the options dialog is like 3 pages of checkboxes for all the various aspects of how they behave).

Also for those who use cloud bookmark/history/tab sync, people might just not want Google specifically to have that data; Vivaldi does its own sync.

MS Edge, Arc, and Sidekick have features Chrome doesn't such as split screen, side panels, and vertical tabs. Likewise for Firefox forks such as Zen.

Customization. There are a lot of bad designs in the original Chrome that can be fixed in a fork