MMH5.5 - Heroes of Might and Magic 5.5

Even back in Heroes 3 (or perhaps even earlier, as my acquaintance with the series began with the third installment), cities were divided into production (army) and income (money) types, with magic sometimes being an additional option.
Moreover, in Heroes 3, this distinction is even more pronounced, as even the starting (production) city is not advantageous to develop into an income city (unlike secondary cities) – this conflicts with the pace of building production facilities.
In Heroes 5, due to the limit on building levels, developing production does not conflict with income in the starting city (unfortunately), but nevertheless, the division into income and production cities is still relevant for secondary cities.
Why should all cities be production-focused and aim for a complete buyout?
And why should all tiers be built and purchased immediately in production cities?
This can only be explained by the desire to extract the absolute maximum from all cities, but then the question arises: why should this be easy or even achievable?