I think I understand how the wicking pad works. I have several sub-irrigated planters and they work great. I like the whole water reservoir concept which is used in my sub-irrigated planters.
But, back to my original question, if the wicking mat is keeping the soil in the seed starting packs moist, is there any difference between that wicking mat and just having a small amount (1/8 to 1/4 inch) of water in the tray all the time? In both cases, I would think, the potting soil will absorb as much water as it can until it cannot hold any more. I don't see any method that only allows the water up to seeds when needed. It's just a matter of if the soil can absorb more water, then it does.

I could be wrong. Maybe there is a difference.
In any case, the way I first was introduced to bottom watering the net cups was to fill the tray to about 1/4 inch full of water and stop. Then you leave the net cups in the tray for a few days as they drink up all that water. When the tray is dry, you wait another day or two for the potting soil in the net cups to dry out a bit, then refill the tray with 1/4 inch of water again to repeat the cycle. Each cycle is about 5 days for me.
That's the way I did it last year and was successful. I am doing it that way this year as well and my plants are growing well.
I have been interested in making my own self-wicking seed starting trays. There are a number of YouTube videos on this DIY project. If you have some of material laying around, you might not have to spend much money at all to make your own self watering seed starting trays...