Decreasing term insurance is often bought to clear a specific debt (normally a repayment mortgage) that is itself reducing over time, with the policy paying out in the event of the death of the borrower or his or her partner.
The term is usually selected to align with the associated debt.
Clearing the mortgage after death is a priority, of course. Indeed, many mortgage lenders will insist life insurance is in place.
The pay-out will mean the family can remain in the property, and the burden of meeting the monthly mortgage repayments will be removed (note that decreasing term insurance is not appropriate for someone with an interest-only mortgage, where the capital debt is only repayed at the end of the mortgage term).
Not all decreasing term insurance is taken out to cover mortgages, though.
Some people also choose this type of life cover because they do not feel such a big payout will be necessary if they die in, say, 20 years rather than within the next 10.