1.) Access to more players means increasing the chances you can find that particular class/job you’ve been looking for your group, raid, crafting needs etc.
2.) Flotilla linkshell would give small free companies a chance to build a reputation among one another. Do you have a small group of good, eager and ever improving players in your free company? Your free company could become known for that or you could come to know a free company like that.
3.) Keep your friends close and keep your free company identity. No need to assimilate into large free companies if you don’t want to.
4.) Escape the tyranny of cross realm PUGs
5.) Exchange of ideas. Different free companies will have different levels of knowledge about the game. You could be the teacher or student.