Yes You are correct, it looks like this.
Tier 1 --> Tier 2--> Tier 3
SEC --> ACC --> Sunbelt
BIG10 --> AAC --> MAC
BIG12 --> CUSA --> Independent
PAC12 --> MWC --> Independent
Yes, it helps keep regional recruiting intact but it also helps greatly with keeping rivalries going even as some teams move up and down between tiers. So, if Miss State moves down to Tier 2, they will be right there with Ole Miss....