One reason they have acting and logging PIC was due to the insurance requirements of multi engine planes. Owners would need a certain amount of twin PIC and could only attain it by having an acting PIC, such as an MEI.
It's much older than that. I did some historical checking and the separation of "real" PIC time from a certain type of time that counts toward pilot certificates and ratings goes back to at least the 1920s.
Here's what I found (edited paste from another thread):
A friend and I were corresponding about some old versions of the current FAR. He came across a hard copy of the 1926 Air Commerce Regulations, and I've seen some later ones. We were going back and forth on the different ways the FAA counted time toward certificates and ratings.
As far as I've been able to tell, the counting of "LOGGED PIC" time is a child of the FAR and didn't exist in the CAR ("Civil Air Regulations") or the ACR before that. Instead of PIC requirements for certificates and ratings, what was counted in the ACR was something called "solo flying." "Solo flying" at the time was not defined as being the only person on board as it is under the FAR. But I think you'll find how it was defined rather familiar. Here's the 1929 version:
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Sec. 61. Meaning of Solo Flying.
As used in these regulations, a person is engaged in solo flying when he is the sole operator of the controls and is in command of aircraft in flight.
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Notice that at least as early as 1929, we have the "sole manipulator" as a concept of how to count flight hours toward certificates and ratings and that back then they were looking for two conditions: manipulator =and= actually in command.
The current version of the rule, still using "solo" instead of PIC ("solo" as "sole occupant" first appears in the 1950s) and dropping the need to actually be the in command seems to make its first appearance in a 1942 revision to Part 20 of the CAR, adding a new rule on logging:
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20.673 Logging of pilot flight time.
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(b) The holder of a pilot certificate, other than a student pilot certificate, may log as solo flight time that portion of any flight during which he is the sole manipulator of the controls: Provided, That he may log as solo flight time only 50 percent of any flight time during which a certificated instructor or a certificated airline transport pilot is in the aircraft serving as an instructor for the purpose of reviewing or increasing such pilot's skill;
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Recall that "solo flight time" is being used for many of the certificate and rating purposes that "logged PIC" is used now.
Change "solo flight time" to "pilot in command" and, except for removing the instruction debit, you have the rule in essentially the same form today as it was 65 years ago. You count sole manipulator time toward certificates and ratings without regard to whether you are in command of a flight.
You can draw whatever conclusions you want from this. And you can argue that the whole thing is stupid or that the FAA should have chosen a term other than "PIC" for "a certain type of time counted toward certificates and ratings." But I find it kinda hard to argue that the way FAA Legal has interpreted the current rule was just some accidental mix-up or something that a bunch of FAA lawyers made up.
Besides, even leaving out the 65+ year history, the formal FAA Legal interpretation of the rule is about 25 years old - FAA has had a long time to change it if it did not reflect what the FAA wanted.
Anyone else notice that the proposed extensive revision of Part 61 doesn't touch it?
