Why do airlines join alliances?

Related questions

1. Can I use Air Canada miles on United?
2. How strict is Condor with carry on?
3. What is a Condor House?
4. How do airline alliances work?
5. Why do some airports not have AK?
6. What are the 3 major airline alliances?
7. Who is in Star Alliance?
8. Is British Airways a United partner?
9. Is Condor airlines owned by Lufthansa?
10. Why do airports start with Y?

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24. #choose
25. #network
26. #lounge