Your computer stores every bit of information and every piece of software that uses that information on its hard drive. If your hard drive gets sick or dies you can lose it all. You can reinstall the software, although that's a major pain, so losing it is not the end of the world. Unless you backed them up, they're gone forever.
Hard drives fail.
They also get wiped by viruses. That's why you want to back yours up. Your computer stores every bit of information and every piece of software that uses that information on its hard drive. If your hard drive gets sick or dies you can lose it all. You can reinstall the software, although it's a major pain so losing it is not the end of the world.