While spy apps may have been created to help parents better monitor their kids, some people use them for totally different purposes. Such devious apps can be used by spouses, relatives, or mistrustful employers.
Your phone certainly includes your pictures, browsing history, GPS location data, notes or voicemails, texts/emails with personal information, etc. That little smart device is actually a portable record of your detailed daily life, so imagine how nightmarish it could be if a jerk gains full access to that information.
So if there’s anyone out there who is paranoid enough to spy on your detailed personal life, they might be using one of these freaky apps. Note that these apps are illegal in some states.