Cities like Kolkata and Chennai were given a lot of importance during colonial rule because

due to huge population, these cities were favoured  raw materials were exported and manufactured goods were cheaply imported majority population in these cities worked in factories, offices or shops had lot of cheap labour available

raw materials were exported and manufactured goods were cheaply imported

Cities had a key role in the economic system of empires. Coastal cities such as Mumbai, Kolkata and Chennai were favoured. From here primary commodities could be easily exported and manufactured goods could be cheaply imported.