Frankly, it’s too easy these days to avoid doing. If you have iPhone, or even any other smart phone/mobile device, it’s simply too easy to not do. On top of that, it’s completely free.
If you have an iPhone, you have the option of backing up your data on iCloud or on your local desktop. iTunes and/or iOS should prompt you from the very beginning to choose to backup or not, if you’ve already passed this stage, you can access your backup data in the iCloud settings on your mobile device. You get 5GB of free data to begin with which should be more than enough to store all of your basic data minus any videos or music you have.
If you don’t backup, you run the risk of losing all of your contacts, emails, documents, messages, etc. that could take tens of hours to re-enter. The time it takes to re-enter all of your settings, content, and data will be a hundred times more than the time it takes to setup and simple backup.