Hopefully, we aren't in the practice of stealing, deceiving, or oppressing those around us, but how often do we confess and make reparations on the day we realize our guilt? Do we make it right as soon as we recognize it, or do we try to negotiate for more time and look for a way out of the consequences? And how often, even when we "make it right," do we add 20% in reparations?
We're all imperfect, and therefore we all sin against God and against those God has put in our lives whether it is by taking advantage of their generosity or outright theft. We sin against them when we are not truthful or tell only a portion of the truth because the whole truth might make us look bad.
But the command here isn't to not do those things (though obviously that is implied), the command acknowledges that we will commit these sins. But when we realize what we've done, it instructs us in how to make things right. Simply giving back what was taken isn't sufficient. There is 20% restitution required above and beyond the damage. There is a dual purpose in this restitution: First, it is to compensate the victim not only for the actual loss, but also for the breach of trust. Once violated, trust is difficult to earn back. Second, it is to drive home the damage our sin does to others and the high cost to ourselves. It's intended to have a corrective impact on our future behavior. The consequences of sin are steep and this penalty is meant to help us see that.
The verses go on to describe the sin offering required in these instances, which we are no longer required to give as Christ is our sin offering once and for all. With this offering, the priest would make atonement for the Israelites' sin, just as Christ has atoned for our sin, and "he shall be forgiven for any of the things that one may do and thereby become guilty.”