The theory behind decreasing term insurance is that a person's need for high levels of insurance decreases with age and certain liabilities no longer exist. A big portion of the decreasing term insurance found today is in the form of mortgage life insurance, which pegs its benefit to the remaining mortgage on the insured' home.
Decreasing term insurance is generally not advisable for someone who has no other life insurance; term life policies can be purchased at affordable levels and provide the security of a level payout throughout.