On some websites you may see an arrow in the bottom left or right corner. When you click this, it jumps you back up to the top of the page with one click. This is helpful on pages with long form content, or infinite scroll, so that you don’t have to manually scroll back to the top of the page.
But, in brief, when you install a plugin or theme, a folder on your server is created.
So if you try to upload the same plugin or theme again, there will already be a folder with the same name, and that creates a conflict – you can’t have more than 1 folder in the same location with the same name.
It’s a good practice to remove unused themes so that you don’t have to maintain them with updates for security purposes.
If a theme is installed on your site, it means all the files still exist on your server. That means the code could still be exploited, even if the theme isn’t active. So you either have to maintain the updates for the theme, or you should just remove it.