Cortana personal assistant can do several things for you. Everyone doesn't certainly need a personal assistant in life, but Cortana can be your Virtual Personal assistant.
Cortana is home to windows phones and windows based laptops. However, Cortana is also now available on iOS and Android.
A feature of Cortana is that you can choose if you want Cortana to respond only to you or just anyone.
Cortana can help you keep Reminders. Using this feature Cortana can remind you with the tasks.
You can ask Cortana to play Music for you
Cortana Notebook understands your interests. You can go and add your interests into Cortana's Notebook.Cortana's Notebook >Interests. Tap the add category and add your interests in the section
A good part about Cortana is that it can keep your phone quiet.