It’s very simple: They want to have rides and lands that the average guest will clearly associate with the MCU, but they don’t want to be chained down by the MCU canon. This “alternate dimension” stuff is simply how they address anyone who is disappointed by canonical plot points not being perfectly maintained throughout the land.
They could just say “Get off my case, it’s a theme park, for pete’s sake!” But saying it’s an alternate dimension has more nerd cred.
Exactly it just gives them the freedom to add any characters they want. That seems to be the main issue with GE and they can't just put whatever characters they want (although I don't think that's why they are going this direction for AC since that was decided way back in 2016). This also doesn't have any specific time frame, its just present day and they can build whatever storylines they want and change things up whenever they want.
And I like the idea they have all the Marvel lands connected worldwide. It doesn't really matter to the average guest but for hardcore Marvel and Disney fans, it will motivate them to try and see all three (CA, Paris and Hong Kong) when they are all complete in a few years time.