Insulating your home, is common sense, it means keeping in the heat that you have paid for. Its a law of physics that "things cool down" so a heated home will lose heat through the building to the outside. If you insulate your home, you are slowing down this heat loss, and holding on to the heat that you are paying for. This will keep your home warmer in winter and cooler in summer.
Most forms of insulation are quick, easy and cheap to install, and a good investment, in that if you do have to pay for the work to be done, you will quickly save the money on your fuel bills.
The diagram below shows areas of heat loss from a 3 bed semi detatached house with gas central heating.