Whether a cell phone is necessary in addition to a landline depends on a person's individual needs. Compared to a landline, a cell phone's biggest advantages are its mobility and advanced functionality.
Cell phones have become more like personal computers and have much more functionality than a typical landline does. For example, cell phones can access the Internet, play music, keep a calendar, take pictures and run a variety of apps. Owning a cell phone has also become very affordable.
On the other hand, a landline has some advantages over a cell phone. A landline is always connected, has better reception and is linked to your address when you make a 911 emergency call.