I use them for several things.
Theme options – header, footer, SM links, other settings and things that are used site-wide
Post type options – I add these options pages to each post type. There isn’t a way in WP to edit the “Archive” pages for a post type. This lets me give the client a way to adjust these pages so I don’t have to hard code anything (except maybe default values).
I also recently created an options page that allows one client to manage the select-able values in several ACF select fields. This gives the client the ability to modify the values that can be selected without actually giving them access to edit the ACF field groups.
I also build custom plugins. Options pages work perfectly for creating settings pages for my plugins.
