First of all, it is much more convenient than regular phone. You can make a phone call wherever you are. If you are ill or see an accident, you need an ambulance to come at once. Mobile phone can save your life.
Secondly,to send short messages is much cheaper than to make a phone call.
Last but not least, nowadays, mobile phone has more and more functions. You can take pictures. You can listen to music by using it as a MP3.
All in all, it is a useful tool. It makes our world far smaller than ever and shorten the distance between people.