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A .psd file is a source file for Photoshop, a graphic editor that is used by most web designers.
The Photoshop source of the theme design can be useful for experienced designers, who prefer to make changes in original theme design, rather than modifying theme files.
Modifying the PSD file will not change anything in the theme, unless you export modifications in the theme files (PHP, CSS). So if you’re not familiar with Photoshop, then you don’t need this file, and it would be easier for you to make the changes directly in the theme files, which can also be modified, without having the PSD file.