I am not an economist so my question would be directed to those who understand economics and government administration. If a country is capable of paying off her debts, why don't they? Obviously , these developed countries I believe are capable of offsetting their debts because they even render aids to other developing nations.
Paying off debt as a country means producing a budget surplus: generate more revenue in taxes than are spent by the government. With how much debt has been generated among these countries, the repayment plans would take some number of decades, with many years requiring large budget surpluses.
Because these countries can print their own money, there isn't an incentive to produce budget surpluses, rather just borrow money and leave it to another generation to pay it back in 30 years.
Secondly, are there any benefits, economic or otherwise of a country being in debt?
There can be, yes. If you believe the debt taken on at any given time will lead to future prosperity, then the debt is an investment. For example, if you're the government and you issue 50M USD out in federal loans for young people to go to college and receive specialized education in engineering and medical science -- that 50M of debt could theoretically pay for itself over the lifetime of these young people given that they contribute to the economy with their specialized education and repay the government for these loans.
But the many trillions of dollars that are tied in debt with the countries you mention are far from investments with a positive return. Point being, not all debt is bad debt.