In summer, the heat from the sun heats up your roof and that heat radiates downwards into your attic. Yes, I know heat rises, but it also convects in all directions. In an attic in summer, you will notice it heating up a lot, especially when the sun is overhead and there is little or no breeze.
That heat doesn't just stay in your attic, but radiates down through the ceiling into the rooms below, heating them up too. If you are relying on air conditioning to cool your home in summer, then the system will have to work harder to remove the excess heat that convects down from above, costing you more in power bills.
Attic insulation helps stop that heat from sneaking into your home by convection. This makes it cooler and puts less strain on your air cooling system, resulting in lower power bills.
Good for Summer, Good for Winter
In winter, the reverse is true. Heat generated from your home's heating system does rise and a large percentage of it will go straight up through your attic into the air outside. Having attic insulation fitted helps prevent that heat loss by deflecting it back down into the rooms below, also saving on your fuel bills to keep the heating system running as it doesn't have to work so hard to maintain a comfortable temperature.
So that is pretty much the main two reasons why you should insulate your attic!