WordPress offers many free themes from their theme marketplace. You can install any of these on your site. However, because they’re free, a lot of websites use them. And this means someone may have a site that looks very similar to yours.
Buying a third-party theme means far fewer users will be running the same theme. Often, third-party themes also provide higher quality designs because larger teams of developers are working on them and being compensated. Likewise, they can often provide support if you’re having trouble launching the theme.