With an Alliance you are assured of a lot of reliable and consistent player contracts for a longer term.
This translates in quicker start up and developing airports, making money quicker, have more SP’s and XP’s and (by bying) GP’s.
Some examples:
When you close down your airport you can make agreements with other Alliance players to take over your contracts. So you can “hop” from one specific airport to the same airport of another player when the first one decide to close down. This provides continuity in the gameplay.
You can also make agreements on short or long term parking of your surplus airplanes when closing down an airport. You can store them at an Alliance airport then. This means you do not have to sell your planes and preserve them for later.
Keeping track of all the incoming and outgoing contracts is an additional challenging aspect which gives an extra dimension to the game. More fun to play
Last, but not least, there is a social aspect. Managing these contracts and constant shift of playable airports means you have to communicate a lot with you fellow members.
On the question “how to get in” I suggest you read the posts in de Alliance section and check who is “hiring” and who is not and on what condition(s).
And you are free to start own! Get an fancy Alliance name, post a description about it and your “terms and conditions” in the Forum and invite people to join!
Hope this helps.
Happy flying