Why should you keep confidential information?

Its always important to be private with things that u value for security purposes. Not everyone who accesses them is genuine and not everyone means well for u. On another hand if it is official you need to keep it safe from say forgeries and being stolen.

Answer

Because it's the right thing to do. Confidential information is confidential for a reason. It shows that you are trustworthy and of good character when you can keep confidential information to yourself and not share it with others that have no business having the information. When someone shares information when they shouldn't it simply becomes petty gossip and no one will respect you or trust you.