Why stats are collected in oracle db?

Some questions with answers on why stats are collected.

1. Why do we collect stats? 
2. How stats collection improve the performance? 
3. Does the execution plan of an already running load gets changed after stats gathering? 
4. stats export and stats comparison 
5. Will there be different plans when there is parallel execution? 
6. Can the stats become stale when only read happens? - No
7. Stats get stale when more than 10% of the data gets modified.
8. How frequently stats have to be collected?

Ref:

http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-optimizer-stats-concepts-110711-1354477.pdf