First of all, a population of 6 isn't a small population. It's a small city a worst. Low population is 1-3. One of the fun things (to me) is analyzing the whys of a main world's existence inherent to the random nature of the generation process and explaining its overall function relative to the subsector and sector. The trade codes definitely help defining a world's purpose. Too often, Referee and players alike only see a world as a dot on the map and a stopover to their next destination.
Also remember the definition of a Traveller mainworld is the designated most important world in a system. There's something about that particular planet explorers and developers decided it will be the place to center around.
Just because a world isn't a garden doesn't make it useless. It may have resources beyond mere colonization or its location in the region has significance. Remember the fourth dimension in planetary development - time. Worlds don't spring fully developed yesterday. They have a reason to have been originally established then evolve over time. They all have a story. Such stories are legion with world generation and stellar relevance. Tom's example is one particular example with a familiar theme, low population inhospitable world having an A or B starport in a system with a gas giant. Such a combination points to reliance on such a port as the main economy. Taken with tunnel vision, it's a good guess but now look at all local star systems. Why would they drop such high level ports there unless there's a need to perform repairs, let alone make, ships that can't be done elsewhere. Maybe it was actually a very successful corporate mining operation (hence the low population) that had the capital to keep upgrading what was originally a D or C class port for mining vessels to bring in more money in the ship business. Another familiar avenue is location. A system could have importance as the link to other systems so it stays solvent as the stopover for ship taking cargo elsewhere maybe, with an A port, becoming a depot for receiving and distributing goods.
So many choices.