Why SeeAllData(True) in Salesforce? Why we use it? what will happen if it is false?

Hello,

IsTest(SeeAllData=true) Annotation
use the isTest(SeeAllData=true) annotation to grant test classes and individual test methods access to all data in the organization,
1) If a test class is defined with the isTest(SeeAllData=true) annotation, this annotation applies to all its test methods whether the test methods are defined with the @isTest annotation or the testmethod keyword
2) The isTest(SeeAllData=true) annotation is used to open up data access when applied at the class or method level

http://amitsalesforce.blogspot.in/2015/09/test-classes-with-istest.html

Use the isTest(SeeAllData=true) means you need to create the test data in your org.

User, profile, organization, AsyncApexjob, Corntrigger, RecordType, ApexClass, ApexComponent ,ApexPage we can access without (seeAllData=true) .
SeeAllData=true will not work for API 23 version eailer .

NOTE:- Always try to avoid use SeeAllData= true because in that case your test class depend on your sandbox data. You May get some issue will deployment.

Let us know if this will help you