It is a way to ensure that everyone gets an equal amount of benefit. As I already wrote, if it were proportional the bit city regions would have way way more voting power than rural areas, and the needs of the rural areas would go unheard, thus them not getting equal benefit.
If by equal you mean proportionate benefit, sure, but in a democracy the minorities get nothing when their ideas clash with the majority, since the majority wins always. Giving them a boost helps balance thing a bit.
I say this again, what US has right now is a travesty of the original idea, there’s other places where this works better. For example, in Spain, the big city regions have more congress seats than regional zones, but it’s not proportional to the population at all, the seat amount is inflated in the rural areas. We don’t have weird ass regions though, the regions are separated in a historical way.