If I have a domain name that does not have privacy protection enabled, is there any point in enabling it now? Or is it useless once the information is out there? I'm asking this because I'm aware that there are services providing DNS history.
Sure there is. All forms of security are about making the attackers job more difficult, not impossible. A physical safe doesn't stop the bad guy from getting the stuff inside, it just makes the attack take long enough to catch them in the act, or increase the cost to get the goods beyond their value. All security controls work like this.
If your cost to deploy a control in time, money, or any other resource, is less than the value it protects, it is a worth-while control.
Enabling privacy protection now stops future folks from looking up your information. Sure, there are historical DNS systems, but some people you are trying to stay private from won't know about that. Further, as time passes, your information may change, and will remain protected, while the history services don't get those updates.
So the question to you is, "Is the time/cost/effort of setting up and maintaining privacy protection worth this 'partial' future cloaking of your identity?" Only you can make that evaluation.
I'd claim that since the question was phrased as "is there any point" the answer is "yes." That point may, or may not be a valuable one, for you but there certainly is some point in turning it on after the fact, despite the existence of historical archives.