Why keep git history clean?

The question boils down to whether you need a history in your VCS repositories or not. You do need it in cases when:

• You need to find the task in your tracker. The task may contain information about why the change was introduced.
• Need to read commit message that explains the change
• Find the person who introduced the change in order to ask him questions about it. Maybe break couple of fingers.
• Figure out when the bug was introduced. Often that's just curiosity, but it may uncover interesting trends.
• Revert the change if malfunction was discovered late
• Catch up with the changes made by your colleagues yesterday. Or while you were on vacation.

So if the history is easy to inspect, these goals are easily achievable.