When you look at a sum of money, whether in a building society or your purse, yes it will be worth less in ten years time but the money in the building society will be accruing interest at compound interest, that is each year not only will the lump sum earn interest, so will the interest already earned and over a ten year period that can be quite substantial.
At the moment interests rates are at a historic low level, but no interest rate is set in stone and in time they will rise again. We can probably all remember when interest rates were around 10% and mortgage rates around 15%. Times, and interest rates, do change. Building Societies also pay higher rates if you can leave the money in for longer terms or you invest them in bonds.
As well, as Frank points out, there are always going to be the emergencies, replacing household goods, cars, large repairs and a whole lot of foreseeable, and unforeseeable emergencies that might arise when having a saved cash cushion is essential.
Long term saving also has its advanatages, more money to fund your retirement - and the new fixed rate pension will take away a lot of the disadvantages of savings when your pension income is low. There is a lot to be said, even then, for keeping your savings close to the maximum allowed before benefits income is affected.
What savings can give you is peace of mind Two years ago our car broke down on a long journey north to celebrate a family occasion. We handed our car to AA Relay, they gave us a lift to a car hire company and within an hour we were on our way again. Car hire does not come cheap, but we could do it without worry because we knew we had a building society account with money in it to pay for just such emergencies.