We store everything on our phones.
From emails, social media accounts, business-related documents, personal photos, to-do lists, and many more. With us becoming even more attached to our devices, the more data we store on our phones, the more vulnerable your privacy becomes.
It's fine if the data stays on your phone. But sadly, smartphones do connect to the internet, and apps may send information about usage and other data to servers elsewhere on the globe.
So no, your data may not be secured as you may have expected.