I confess that, years ago, I didn't pay much attention to closing wheels on planters. I'd set an "average" down-pressure on the spring adjusting handle and never change it. Since then I've learned both the importance of properly adjusted closing wheels, and how to do it.
In short, the angle of closing wheels is designed to press the sides of the seed furrow closed without actually putting pressure on the soil above the seed. Too much downforce on closing wheels packs soil around seeds and inhibits both shoot emergence and growth of seedling roots. Insufficient downforce doesn't firm the soil enough to remove air pockets, which can influence moisture uptake by the seed and delay germination. "Perfect" settings vary with soils and soil conditions, but with the planter lowered, I like to be able to lift by hand each closing wheel frame an inch or more above the ground without excessive effort. If I can lift the closing wheel frame with one or two fingers, there's not enough down-pressure. If I can't budge the closing wheel frame, there's too much down-pressure.
That's a starting point. What really matters is how the closing wheels leave the soil beside and above the seeds. My rule of thumb is that when digging seeds behind a planter, I should be able to easily dig down to the seed with a digging tool or small screwdriver. If the soil comes up in clumps or clods, there is either too much downforce on the closing wheels, or the closing wheel frame is out of alignment and one of the wheels is running directly over the seed furrow, packing soil on top of the seed.
If seed furrows aren't getting closed and seeds are exposed to air and sunshine, I'll say this about that: If you're planting through a small wet spot, increasing closing wheel down-force enough to close the seed furrows in the wet spots will probably result in too much down-force in the rest of the field. If you're leaving open furrows across much of the field, it's probably too wet to be planting.
I now understand that closing wheels on planters are actually a very important part of planting. Remember when you were a kid helping your mom or grandma plant their garden, and how carefully they used their hands to gently firm the soil around the seeds, or the roots of each young tomato or onion plant? That's what you want your planter's closing wheels to do. With careful alignment to ensure the wheels are centered on either side of the seed furrow, and just enough down-pressure to firm (not pack) the soil around the seeds, you'll give each seed a great start toward optimum yields.