Why do people buy preamps if they can directly connect amp to stereo source/phone/PC/tv?

Either way, you need a preamp - either it's built into your DAC or CD player.The preamp is not just something to switch from one source to another (which is an easy part).  The harder part of the preamp is to control the volume of the source which can be pretty expensive (not to mention tube vs ss).
The question is: which is better - the preamp in your source or some external preamp?
Eventually cost comes into play.  Therefore an external quality preamp is always better than a built-in preamp of your source because the source player was probably designed for a price point.  A good external preamp usually sounds more dynamic because it usually employs better power supplies. 

This argument is similar to which is better: an integrated amp or separate?  Sort of the same thing.  With separate you can optimized for its specific function.  With integrated there is some type of compromise.
It's hard to have the best DAC and best preamp built into one chassis.  But it's a lot easier to have the best DAC and best preamp separately.

Multiple sources, plus not all sources have a high enough output to drive a power amp, the signal needs to be amplified first. For example, I once connected my portable Digital Audio Player to my stereo amp, but it sounded very hollow and was struggling, you can basically tell it was congested and running out of breath to use an analogy. 

For those that like tube coloration (or distortion. for you detractors), messing around with tubes is a joy.  For those that prefer more sonic honesty, you will need to spend considerably more for tubes and may well find sonic bliss with a less expensive, but well specified solid state unit.

I feel that the tubes are more honest, generally speaking; solid state has a coloration of brightness and hardness.

Generally for sonic improvement.