Because it’s an appeal to the unconditional covenant love and compassion of God [51.1]. David is flagrantly open and vulnerable concerning his sin. David acknowledges that his sin is against God and God alone [51.4]. The need for purity and holiness before God is clear [51.7-11]. He knows that broken bones will rejoice [51.8]. It reminds me that those things which our Lord delights in are humility, contrition, and joy in His righteous salvation [51.12, 51.17]. Another aspect of this that provokes my spirit is that David prays these things for the sake of others… “Then I will teach transgressors Your ways” [51.13]. And for the sake of God being honored… “Open my lips that my mouth may declare Your praise“ [51.15].
I hope that in doing this, I’m examining myself so I don’t eat and drink unworthily [1 Cor 11.27-29]; I’m remembering Jesus’ work on the cross [1 Cor 11.24]; and I’m proclaiming the glory of His death until He comes [1 Cor 11.26].