God didn't need 6 days, He deliberately used 6 days and rested on the 7th. And He didn't rest because He needed to rest.
Everything God reveals about Himself has a reason, often to show something about Himself or to set an example. For example, why would God ask from Abraham to bring his only son to a mountain and sacrifice him, not because God wanted Abraham to kill and sacrifice his son, but to show something of what was going to happen 2000 years later.
Why would God allow a nation He chose and loved as the apple of his eye, to become slaves in Egypt for 400 years before being liberated? To show something what was going to happen 1500 years later.
Why would God created the earth in 6 days and rest on the 7th, if He could have done it in a split second, to set the example for us, work 6 days and rest the 7th, that's what God commanded to us, while He made us and knows what our bodies need (it's confirmed by secular science that a 6 day work 1 day rest rhythm is the most adequate for a human). And besides, more importantly, also this points to God's plan with mankind. The book of Hebrews explains how the resting on the seventh day ultimately has one goal: profetize how those who are born again enter into their Rest. God could easily have us made without a need to sleep or eat. The fact that we need to rest is an anology how we need a Rest on a whole other level, Jesus Christ.
In the end: everything points to Christ.