Here is my opinion on your questions.
How much important is this platform in a Magento developer career?
I think MageStackExchangeange is the best and very useful platform for Magento developers. This platform is very useful for newbies also very helpful for the experienced developer of Magento to find solutions in a very quick way.
Is it must join to become expert at Magento?
No, But if you join MSE. You can learn Magento development very fastly by other answers and also by solving other problems.
Does it help a developer in getting a good job in the well-reputed company?
Yes, If you have a good profile in MSE. It definitely helps to get a job in the well-reputed company.
Do you think a Magento developer can become expert without joining this platform?
Yes, But as said before if you join MSE then you can learn Magento very quickly by posting your problems and by solving other problems.