I'm a PhD holder.  Reason why I did it?  Hmm.. so that people can call me Dr.  And .. people are more responsive when you say you are Dr. so and so

I am starting my PhD next year and I can't be bothered to go through the rest of the posts due to my slow connection, so I'll just go ahead and type
(I have done lots of research and asking my PhD friends prior to taking this step)

1.  DO NOT DO A PHD THINKING OF THE EXTRA MONEY, except if you are already working as a lab assistant/helper in the academic line and wish to further your career within it (you can probably have a better shot at funding, being an insider). You will earn more money working those 3-5 years and end up in a better salary position at the end of it.

2.  "Prestige" is a side benefit - do not go in solely because of that - a PhD is a long and costly road with many nights of hardship - not worth doing it just for that - in the end you are just another scientist

3.  you didn't know what to do with life & you had a sponsor - I suspect quite a number of PhD students do this - however, a lot of them also dropped out precisely because they started their PhD for this reason

4.  your father told you to - ... not sure what to say to this

5.  These are the best reasons for doing a PhD
(i) Wanting to discover something that benefits the world
(ii) (Really) Want a job in the academic line
(iii) Strong interest in your subject + hate having a boss hovering you all the time + hate routine/bodoh-type jobs

In short, a PhD is like a job in research, except you don't get paid in money, your "salary" is the experience, the results of your PhD research project (which may potentially change the world, gain you *ahem* your supervisor *ahem* fame)  and of course, your doctorate degree and you can have everyone call you "Dr."

That is all folks