Simply put, it's one of the ways that we keep a good relationship with God by following the commands he gave us. Most people are somewhat familiar with the 10 Commandments, the fourth being:
"Remember the Sabbath day by keeping it holy. Six days you shall labor and do all your work, but the seventh day is a Sabbath to the LORD your God. On it you shall not do any work, neither you, nor your son or daughter, nor your manservant or maidservant, nor your animals, nor the alien within your gates. For in six days the LORD made the heavens and the earth, the sea, and all that is in them, but he rested on the seventh day. Therefore the LORD blessed the Sabbath day and made it holy." (Exodus 20:8-11, NIV)
Working 7 days a week isn't beneficial for anyone, company or customer. Following the model that God set for us in his act of creation and later commanded by etching in stone for us, is not only beneficial, but right.