The intended purpose of reshade is to upsize/zoom images without degrading their quality. It was meant to overcome the problems with other algorithms available (including the ones in Photoshop) that create blurry enlargements. The algorithm was implemented to work well for small upsizes of about 2 times the original, but eventually it turned out to do a good job for any resizing factor.
See more details about reshade.com in the about section or view the articles below.
What it offers, how and why to use it.
Practical results of reshading.
Where to find wallpapers and how to improve them.
How image enlarging works.