Rambam in Hilchot Temura 4:13 Discusses a law in this week’s Torah portion of redeeming santified property:
The principle behind these laws is that the Torah descended to the bottom of a person's thoughts and the scope of their evil inclination. For human nature tends to increase ones property and attach importance to ones money. Even though a person made a vow or consecrated something, it is possible that they will reconsider, change their mind, and redeem it for less than it’s worth. Hence the Torah states: "If he redeems it for himself, he must add a fifth." Similarly, if he consecrated an animal in a manner that its physical person becomes consecrated, he might reconsider. In this instance, since he cannot redeem it, he will exchange it for a lesser one
Therefore, the Torah removed that option, forbidding all exchanges and penalized him that if he made an exchange, "It and the animal to which its holiness will be transferred shall be consecrated.
All of these ordinances are to subjugate one's evil inclination, improve one's character and make ones conduct upright.