A person performs ablution before prayer but then enters a state of ritual impurity before praying. Because the original purity has ended, the condition necessary for valid prayer must be restored.
Renewing ablution is required because purity has been lost while the prayer still calls for it—not simply because time has passed since the earlier ablution.
Conversely, someone already in a valid state of purity when the prayer becomes due need not perform a new ablution merely because a new prayer time has arrived. The intended condition is already present.