I was just having a quick read on TFwiki to see where or when the concept originated, it was Simon Furman in 2003 who first suggested the Multiverse idea and dimension-hopping as a way to explain multiple versions of the same character over the years (toys and stories) when he was doing the 3H Universe comic. From there it seemed to be an easy way for others to get around questioning the use of certain characters in a different storyline (particularly since Transformers has had many, many, many storylines/toylines over the 30 years... and any relation between those stories is often retconned rather than pre-planned).

So Furman set the precedent of Unicron being a singular entity, and a couple other characters in FunPub comics in the next few years, but when The Fallen showed up in the TF2 movie, questions were raised about if it is the same character as the comic Fallen... since they both had similarly unique betrayal back-stories.
This was also around the time of Hasbro creating their "First 13" lore... which included The Fallen, and these 13 were then claimed to be Singularities (in that only one exists, and if you see a storyline with that character in it, it is the same as in other stories).
To complicate things though, one of the people who had been working on the early FunPub comics (Forest Lee), who developed the Multiverse concept even further to tie together more Transformers stories to each other, went to work at Hasbro as a copywriter (character bios and promotional materials)... so he may have influenced the idea of Singularity characters and Multiverse into canon across the brand (not just within FunPub's universe).

A quick comment by him on a Hasbro panel at BotCon was later expanded to a rather complicated, yet oddly scientific, explanation as to why "Singularity" characters can exist and "die" in multiple forms in multiple universes, but still be the same single character.
But even that has had some contradictions in the past (Fun Pub having a Shattered Glass Alpha Trion, before Hasbro's retcon of the First 13 as being Singularities).


I like the idea of finding a way to tie all the Transformers story universes together in some way so that a toy collector can play with different toys more "plausibly", but I think this "Multiversal Singularity" idea is a complicated method that was too much too late. It should have been kept simple, as just Unicron (and maybe Primus) early on as the universal "god", and all future uses referred to the story of the first version.
Hasbro is supposed to have some sort of "executive producer" control over their brand, particularly when it comes to approving stories by their license holders.

I think many of us who have been fans longer than 3 or 4 series, have created our own reasons for them to be related, particularly if we have toys from each series and play with them. Mine was a faulty Spacebridge that would cross dimensions, allowing characters from each to interact with other dimensions (like Gen1 dimension, Beast Wars dimension, JP Gen1 dimension, Gen2 dimension, Armada dimension, etc).