Nitro has a lot of useful perks for people who spend a lot of time in Discord, such as the ability to upload larger files, send longer messages, and customize your profile, but I’d argue that you don’t have to be a power user to appreciate aesthetic changes. It’s easy to imagine people wanting themes but not being able to justify such a high price tag for an app that they just use casually.
Discord does let you preview the themes to see what you could make the app look like if you were a subscriber. (You can access the preview pane by going to Settings > Appearance, then clicking the “Preview Theme” button.) In my opinion, there’s a nice mix of pastel-colored light themes and cool / moody dark themes — there’s even a “blurple” one that almost feels like a Verge theme.
For people who don’t want to pay for Nitro, there are still ways to theme Discord. The BetterDiscord project has been working to enhance the app for a while now, and the list of improvements includes a theming engine. And while it lets you create truly custom themes rather than giving you a bunch of premade ones to choose from, it’s admittedly not quite as user-friendly as the Nitro version.