Your digital files have been color corrected and post-processed with great care in Photoshop. In the files, "metadata" is embedded information that tells a lab how the images should be printed and color calibrated to match what we saw on our screens. A professional level lab will read that data and print a product for you that is very close to what we would print for you here. A consumer level lab (Walmart, Costco, etc) operator is typically not trained to read this data or adjust the machines to appropriately print the files.
Printing with our professional lab ensures you will receive a product that matches what we produced for you here in studio, and it also ensures your prints will be of archival quality and last for generations. A quality, printed is image is an heirloom.