Desktop environments are inherently very personal things and so what one person might find great, someone else might consider unusable. Themes are a good way to add room for user preference without forcing the user to change desktop environments entirely.

For example, before libadwaita screwed it all up I used to like to apply a custom theme to GNOME to cut down on the egregious padding everywhere, making the UI a much better fit for non-touch desktop OS use on a small laptop screen. There were several themes that accomplished that very well.