Affiliate Marketing Secrets

I’m talking especially about General Motors, before they cut down. Many have 3, but why did GM of America ever have so many?
Up to 2004, I believe there was Saturn, Chevrolet, Pontiac, Oldsmobile, Buick, Cadillac, Saab, GMC, Hummer. Forgive me if I’m missing any.. I know the strategy worked in the company’s heyday, but why in the first place?
They got rid of Oldsmobile in 2004 and more recently cut down to Chevrolet, GMC, Buick, and Cadillac. If it weren’t for Buick’s popularity in China, I’d say Cadillac could have more entry level Buick models, and Buick could be virtually expendable. GMC has almost all the same trucks as Chevrolet’s truck line up.. Why not just add trim levels to Chevrolet’s trucks? Hypothetically, couldn’t GM very well be Chevrolet and Cadillac?
Now, I’m not a business expert, but some of it didn’t make sense to me.. They said these 4 core brands “could build” all their models, i.e. GMC takes on Hummer’s niche, Chevrolet takes on Saturn and Pontiac, etc. But why did they have to worry about a division “being able” to build their cars? Why couldn’t they just change the freakin’ names? It’s a fair point.. And on the other hand, how does having fewer brands help an automaker? Is it just to solve an issue of identity and have less branding required? Hypothetically, why couldn’t they have taken 3 Pontiac plants, had one produce Pontiac, and the other produce two brand new but similar brands? It might sound ridiculous, and I don’t see the point to having a bunch of different divisions, but why is it such an issue? Couldn’t you just take 1/3 from one division and tell them they’re a different, but similar brand now?
So what was the point of all the brands in the first place? Was it like you start out in a Saturn, then you own some Chevrolets, then a “sportier” Pontiac, then you own a Oldsmobile in your middle-aged years, then a Buick, and finally a Cadillac as you get older? And why was there SO MUCH overlap? Many Chevrolets, Pontiacs, and Oldsmobiles were EXACTLY the same – think Chevy Venture, Pontiac Montana, Oldsmobile Silhouette, and even Buick Terraza. Why?! Then you have somewhat upscale Chevrolets like the Impala, but entry level Buick Skylark.. I just don’t get it. They built in each others’ supposed niches. And for the life of me, I don’t think I’ll ever figure out what Oldsmobile’s place was…
P.S. I like a lot of GM cars actually, and I’m not knocking the company, but I just don’t get it.