The thing is that even with a lesser series like Star Trek, the "filler" might not be particularly memorable, but it still serves to familiarize ourselves with the characters. That way, when an important episode comes around, we already know and care about them.
For example, Star Trek Discovery season 3 started with the Discovery crashing after coming out of the wormhole to the far future. In one of the early episodes of the season, the ship's helmsman had some confidence issues because she felt she should have been able to handle that better. I did not care one bit about that, because the show had not taken the time to let us get to know the character.
In Deep Space 9, on the other hand, the third season also starts with something of a shift in the status quo. The crew is taking the new Defiant into the Delta Quadrant to make contact with the mysterious leaders of the Dominion, the Founders – and when they do, they learn that they are the same species as the orphaned station security chief Odo, which is of course overwhelming for him. And we care about his reaction, because we've gotten to know Odo over the last two seasons. We would not have gotten that if each of those seasons had only been 10 episodes.
