Given that so many of the details of our digital lives are either with us (on our smartphones) or easily accessible (via the web), you should be doing everything you can to protect that information and data. On iPhones and iPads, data is largely kept in a vault, sealed behind strong encryption and (hopefully) a strong password. Even if the device is lost or stolen, chances are good that encryption will keep data safe. (That vault is secure enough to frustrate even the FBI.)
Although iOS devices are designed and built to be secure, data is also stored and accessible online. With security breaches occurring routinely, your data is vulnerable to anyone in the world with an internet connection and a halfway decent browser. If a breach occurs and thieves gain access to your email and password, they can easily reset any account linked to that email, change the password, and lock you out of your own data.