Well, that is a good question.
Alas the answer isn't as simple as it seems.
Ok, my understanding of why we have different datums.
I may be wrong, but I am sure the idea is correct.
OK, maps originally didn't have Lat/Long. This was a later addition.
And there are MAPS and there are CHARTS - I am still coming to understanding the specific differences but anyway.
I'll call them maps when they could also be CHARTS, but for the sake of simplicity....... MAPS shall suffice.
Say it is WWII. Your side have maps and the last thing you want is the enemy to know where you are.
This can be reduced by using a custom DATUM. Yes, I know: Millitary maps use a different system - GRIDs etc.
Indulge me.
So unless they know the datum and where it is, your maps are not as useful to them as they could be.
Ok?
Now, Let's say someone wants to map Oz. They use/pick/create a datum and use it.
Someone else in Yank land wants to make a map of America. It would be "silly" us using their datum, and/or them using ours.
So they have their own datum.
Likewise for Europe.
Then there are the POLAR areas. Normal Lat/Long don't work too well there.
So, let's say everyone used ONE datum. It is 1900. Or when ever.
The accuracy of the datum is technology dependant. As "back then" it isn't as good as it is today, don't get ahead of where I am going.
So, everyone uses that datum and make their maps with that datum.
There will be inherant errors with any map/datum system. Nothing is PERFECT. I don't undertand the shortcomings of different datums and/or the errors between them.
Then technology improves.
Errors are detected. A new datum is defined. Everyone starts using the new datum on the newer maps.
Therefore all maps MUST have their datum specified on them. As well as that the dates for which the map is valid.
There are other things which MUST/SHOULD be on a map but that's outside the scope of this post - and my knowledge at this point in time.
Another example:
Say you want to map out your property - you have a BIG farm out the back of NSW.
It isn't for anything legal. It is just for your benefit.
You can set your house/home as the datum and refere all points to it.
Getting/using the "official" datum is too much work and/or not applicable to your needs on the map you are making.
Sorry I can't go into all the specifics, but it has been too long and some things I never learnt.
But I hope this kind of explains why we have so many datums.
:)