[PvK]

Another reason would be to set up a military base, with defenses and facilties to resupply, repair, and refit ships, store units, etc. In this way, you can move towards taking other player's developed colonies, and also expand the area over which you can operate your fleet in strength, particularly since more powerful ships in Proportions have serious tradeoffs between speed and range.

But I think oleg was referring to the planets that you try to develop to be as productive as possible, in which case it can make sense to concentrate on only a few at a time.

PvK