system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] Does it realy make sense to give worlds a special task (Forgeworlds, Hiveworlds, etc.) instead of dumping everything on one planet?
gpt
Yes, for various reasons. When you can't produce enough product in your workshop, you build a factory. When you can't produce enough product in a factory, you build an industrial park etc. You keep everything from the factory lines to the labourers in one place so nothing has to travel far and that way you maximise output.

At the scale where the imperium needs product, it makes sense to dedicate entire planets to industry or some other purpose.

To nuance it a little, no every planet is mono purpose. The imperium contains millions of inhabited worlds with an unimaginable diversity. Some are at a stone age level, others are virtual utopias and everything in between.

Some worlds however have such a unique character that it would be a waste to use them for anything else. Let's say there's a world that has a perfectly habitable atmosphere and it's wedged between suns at a distance that gives it a temperate climate an daylight almost around the clock.

Are you going to shove it full of cities or are you going to make the most of all that sunlight and grow 4 planet sized harvests a year?

If you find a planet that is almost entirely composed of minerals with no biomass whatsoever. Are you going to terraform it and go through tremendous effort and expense to turn it more 'earth like' or are you going going to capitalise on all those valuable minerals to make it a planet sized factory 
line?

Hives are another good example. Hive worlds aren't build intentionally. It's just what you get after thousands of years of 40k style urban development. The cities keep getting bigger, the planet keeps getting more ruined. In the end you just have a planet with nothing more than massive urban centers in toxic wastelands.

But yeah, in short it does make sense to have a smaller number of hugely productive industrial worlds with a massive product output than it does trying to collect a little bit of output from millions of worlds and redistributing that.

It's not like every 40k world is self sufficient. Even though plenty are (millions of worlds, staggering diversity remember).