Why should you not overuse 'could'?

Could is often overused or misused. The correct use of could indicates possibility. It means something might happen.

The election campaign could get ugly this year.

In many instances, people use could when they actually mean will, which is a more definitive word.

She could go to the store and buy one for herself.

This has a completely different meaning when you say:

She will go to the store and buy one for herself.

Only use could if you're describing truly conditional situations, or deliberately trying to be vague. If you're describing something which is definitely going to happen, use will.