Answer:
The Confederates had their own flag because they had different beliefs than the Union.
Step-by-step explanation:
The Confederates wanted more freedom and the ability to have slaves, and the Union didn't, so they seceded from the Union. This succession gave them the opportunity to create a new flag, as the Union already had one (what is now referred to as the American flag). The Confederacy kept the same basic principles, but changed the flag to one that better reflected them.