The source file is needed when you want to get the best print outcome. The artwork was made in layers and you can make your adjustments easier. It requires Adobe Photoshop to use it.
Why is the Source File important?
Depending on the size you want to print on, the print company will often cut into the design rather than adjust the design for the best outcome. This can easily be avoided when you print with us as we always arrange the design properly by creating extra content for the edges, so the print will not cut into it.
Here is a cut into the design print example: Here
Here is the adjusted print example: Here
The cut into the design print only happens when you print on canvas & sides of the canvas. So posters and other material prints are not affected if you don't have to print on sides.