There are many acceptable reasons to preach on giving: to see giving increase; to help people understand what the Bible says about giving; to provide biblical instruction on stewardship or money management. There is nothing wrong with these goals. But these goals are secondary to the goal I have in mind.
When I read Psalm 30, I get the picture of someone singing a praise song to God for his deliverance, his compassion, his help, and his restoration. I get the sense that David knew deep down in his bones that he did not deserve to have God deliver him and restore him so compassionately or so helpfully. Nevertheless, this is what God does. God is in the business of taking really messy and really broken people and restoring their lives as he delivers them from their self-inflicted graves.