To quote a previous example £8,900 will buy £10.00 a week in extra pension. That is £520 a year, a return of 5.8%, which is a lot better than you will get putting the money into any building society or bank savings account.
In addition the money you get will rise every year when the pension rises so if the state pension increases by 2% the following April the return you get will go up from £10.00 to £10.20. If the same happens again it will go up from £10.20 to £10.40 under the usual pattern of compound interest and so on and on.
I appreciate that the capital is gone for ever as with an annuity, which if the capital is necessary to you, you cannot afford to do, but if you are relatively young retiree and are reasonably optimistic about your longevity then it could be a very good investment.
I was made redundant into early retirement in my 50s and used most of my redundancy money to buy extra pensionable years. I have now been retired over 15 years and if I live to be as old as many of my family I could live into my 90s. Compound interest means the return I am getting on that money is already more than 10% per year and will continue to rise.