When you upgrade to Windows 10, Microsoft tries to make things as painless as possible for you. It's the reason why your old Windows installation is kept nice and safe in its own folder when you upgrade to Windows 10. That's great if you want access to your old stuff, and even better if you want to downgrade.
If you're limited for space however, you're going to want to get rid of it. If you are sure you want to keep Windows 10 and you have no intention of rolling back, then you can delete your old Windows installation in order to make room on your storage drive. This unused Windows file can be upwards of 15GB, which can take up a significant chunk on a Solid State Drive. [More]