It’s not that God needs to know; it’s that we need to know
What if one of the purposes of faith through obedience is practicing believing God and Christ?
If we can believe enough to do things like paying tithing and obeying the word of wisdom, and we practice doing those things regularly, through faith, maybe we will believe in the power of Christ’s atonement when things get really hard and complicated and our brains aren’t able to think things through or comprehend through pain and hurt. What if practicing our faith muscles isn’t to prove to Heavenly Father, but to help us believe that Christ can save us in anything as we believe in Him and believe Him.
It’s not that God needs to know we can obey and believe Him. It’s that we need to know we can trust Him, especially when times are tough.