Without insulation in your home the heat and cooling goes through your walls, ceiling and floors and goes out those areas to the outside.
So without insulation you're heating your house and the heating escapes faster out the walls, ceilings and floors so that causes your furnace to come on more often and run longer to maintain the heating in your home.
The same is true with cooling when you're cooling your home the cool air can escape through the walls, ceiling and floors and the air conditioner has to run longer and more often to maintain the cool temperature in your home.
When the heat and cool air stay in your home the furnace and air conditioning doesn't have to run as often which saves you money on your electric bills, gas and other heating bills.
So if your home doesn't have insulation you should add insulation which will make you more comfortable and save you lots of money as well.
Also if you have old drafty windows I would recommend you update those windows to new energy efficient windows as well.