If you live in an older house you may well not have double glazing throughout. Often if there has been a new extension or a window has been replaced it may be double glazed but there may still be many windows that are only single glazed.
It can be costly having windows replaced and often people only do it if they are leaking but it can actually help save you money in the long run and add extra security to your home. Doubled glazed windows are very hard to break in through unlike single glazed. This can often be enough to deter a burglar in the first place.
Double glazing makes any room more energy efficient as it will help to keep the heat in. This means during the colder months you will not have to have your heating on for as long or as high as the room will warm up quicker and stay warmer throughout the evening. Many window companies offer finance to allow you to pay for your windows over a certain amount of time and you should also receive a good warranty to cover them for years to come.