This article explains how to apply a different GTK 3 theme to specific applications (a theme other than the global/desktop GTK theme). So why would you need to set some applications to use a GTK3 theme different than the theme set for your whole desktop? There are cases in which an application doesn't look as it's supposed to / looks broken with a particular theme, but you want to continue using that theme for the other applications on your Linux desktop. Or maybe you prefer to use a particular theme only for one or two applications, while using a different theme for the rest of applications, e.g. you could set text editors to use a dark theme, while using a light theme for all other applications on your desktop.