What is SITA?
 

I'm newbie in aviation industry. I search about several software company in this industry such as Amadeus.
But I don't so far know what is SITA?

• Why all airlines and airports use SITA services?
• Which products or services cause this usage?
• Why SITA gather all flight and pax information all over the world?
• ...

Best start here - https://en.wikipedia.org/wiki/SITA_(company)

I read it before, but it is not my answer.
SITA provide some software and services like other companies. but Why all airlines are the member of SITA? What is sita membership?

http://www.sita.aero/about-us/who-we-are/sita-history

Airlines, airports, aircraft, ground handlers, governments, air cargo, aerospace, air navigation service providers and international organizations all use our information and communication technology (ICT) solutions, relying on SITA's expertise to keep the wheels of the industry in motion.

Owned by the industry, we're guided by around 400 air transport owner-members around the world. And we’re steered by the expertise and experience of the SITA Board and SITA Council, comprising over 45 air transport industry CIOs and other IT professionals.


http://www.sita.aero/about-us/sita-membership

Our membership base includes airlines, airports, airport-based organizations, and air traffic management with 95% of all international destinations and over 13,500 industry sites connected by SITA‘s network.

That's a lot of words that really say nothing. "We're a bunch of folks who all work together to do great things, with technology and expertise."

I suggest starting at Solutions & services to read about actual soluitions such as

It's British Airways new economy class offering - Sit In The Aisle.

You mean that the telecommunication infrastructure is the reason of this universal adoption?
If so, why all pax data store on sita? As an example internet is the big communication infrastructure but all packet is not save on a data store.

Almost 70 years ago the airlines at the time started to recognize that there was a need to communicate with each other in a standard way for scheduling, airport operations, etc.

There was no standard way to do so at the time, so a bunch of airlines go together to form SITA.

SITA's operations today are much more than just a communication network. They provide applications for airline operations, airport operations, airplane in-flight communications to airline operations centres, etc.

SITA is a company that operates among other things a network infrastructure tailored specifically for the needs of the aviation industry. SITA is not like the Internet which is made up of thousands of company networks that are inter-connected. SITA does not store any PAX data except for those airlines that outsource their passenger booking operations to SITA.

Thanks for your fruitful information.