So this is how the whole thing started:
Imagine you got your Concealed Handgun License. You took an 8-hour class, did some live fire, took a test, passed a background check, got fingerprinted, had digital photographs taken, your name added to a registry, paid for all of the above out of your own pocket, and now you are a citizen with the lawful ability to conceal a handgun on your hip and have been vetted by your local department of justice/law enforcement.
So you clip your holster onto your belt, holster your Glock 19, and go about your day. You have to walk to the ATM down the street, and it's a windy day. A police officer drives by and as he is looking at you, a gust of wind blows up the shirt that had been concealing your Glock. Now you aren't concealing anymore-- you're openly carrying. The police officer, depending on his temperament and personal beliefs on armed citizens, can now arrest and imprison you.
The same thing goes for if you bend the wrong way to tie your shoes, or slip out of your car seat and your shirt rides up and exposes the pistol's grip, etc.
Originally, the Open Carry Texas movement was started to protest the law regarding accidental exposure. The movement has been coopted-- either by newcomers or by mission-creep.
Ultimately I support the right to openly carry a pistol or long gun for people who already have their concealed handgun license. Logically, it's the same reason why I am against waiting periods especially for people who already own guns. If you own 10 guns already, why do you need to wait for a "cool off period" to buy another one? If I am already vetted by the government to conceal a handgun, what's the issue with carrying it openly?
Of course, I think tactically open carry is a bad idea-- but it certainly has it's uses. The guy standing in line at a bank open carrying is going to be shot first by the bank robber because he clearly poses a threat to the robber's goals, for example.
Some of the good uses are for things like outdoor sports (hiking, camping, etc). It's far more comfortable to wear a holster that goes on the outside of your clothes than to conceal one, and it's much faster to draw should you be forced to use it. Another application is for people who sit down for long periods of time. Sitting with a compact-pistol at 3 O'Clock is not comfortable and can even cause your holster to shift or fall out.
So there are just a few practical reasons, and I hope I explained the history of it well enough.