While convenient and cheap, it's not always a good idea to bring the ratty old sleeping bag that's wallowed in your garage for decades on your camping trip. Along with being comfortable, you want to make sure your sleeping bag is climate appropriate for where you'll be camping. Mild, wet weather environments call for one category of sleep gear; frosty, arid conditions call for something else entirely.
You don't need a new sleeping bag for every camping endeavor you participate in, but do make sure that what you're bringing won't be hopelessly inappropriate for the sort of camping you're planning to partake in. Take the chilly scenario. Sleeping bags typically come with temperature ratings -- that's the coldest spot on the thermostat you can expect to be comfortable at while wearing a good set of long johns. In summer, a sleeping bag with a 35-degree-Fahrenheit-plus temperature rating is usually sufficient (1.7 degrees Celsius). In cold weather, your sleeping bag should be rated more in the minus 10 to 10 degrees Fahrenheit range (minus 12.2 to minus 18.3 degrees Celsius). It's by no means a guarantee you'll be super snuggly, but it'll go a long way to making sure you don't freeze your butt off in the great outdoors.
You may also want to consider some extras that could lead to better rest. Sleeping pads are a great way to stop those dratted pebbles and sticks from giving you a sore back, elbows or knees. They can help insulate you from chilly ground temperatures, too. You'll find all sorts of sleeping pads; some are made of foam, some are filled with air. Heck, you can even spring for a cot.