In a sector that isn't developed, finding planets is worthwhile as you can trade knowledge of them to the other races for relationship bonuses. If you explore a gas giant, which can't be colonized, then you can't trade it as the races do not colonize those.
In a more developed sector (based on your personal choice during sector setup), it seems like all of the planets except gas giants are already explored. The only reason to explore those is the same as exploring an anomaly or space junk. They are a treasure chest essentially. Perhaps gas giants should be "mined" for gas or something. I don't know why else they exist, really.