Reduced File Size
One of the most crucial reasons to flatten your design works is it greatly reduce your file size. Everyone loves having a small size as it makes:- sending it through email possible and can be done swiftly
- loading file on older computers a lot faster without too much problems
- less likely to have printer pooling error
Protect Designer's Intellectual Property
The difference of having a file flatten is it makes it a lot harder to steal your work. A design work is normally comprised of multiple layers of image stacked on one another. As the file is flatten, it is impossible to retrieve an image as whole as partly of it may been covered by another layer or object in the image. People may still be able to steal your work but it will be way way harder and some people would just give up on doing so.
Another benefit from flattening image is it makes all your Layer Styles and Adjustment Layers invisible to the eyes of other people. The knowledge of manipulating your own design work is what makes you unique in the market and people wants you for your unique work. Imagine what would happen if everyone know your secrets, skills and techniques? That makes you are not unique anymore and you are going to lose your clients because they have the options of getting the similar design work at probably a cheaper price.