Why they are designed if they don't bring income or nothing positive?

Some people want to watch the world burn. Back in the olden days when fewer planets were locked down by controlling factions and build wars were more common, people would sometimes build them to intentionally ruin the stats of an enemy's planet. Others I think, weren't even all that interested in what they did to the stats -- they built them for the RP value (which used to be much more prominent, before spreadsheets and calculators took over the game).

Also originally this game was going to have slaves and the means to make/acquire them. The game has gravitated a bit away from that, but they are a relic of that past.