Why should tilth not be too coarse?

Die hard gardeners, who do things properly, get their digging done in winter, so that the frost can finish the work of breaking up the vegetable patch. Rough clods are left through the cold months so that by spring all that is needed is a light forking over, followed by much raking, then trampling - in the heaviest of boots - more raking and finally sowing. This produces the Perfect Tilth - ground which has been tilled to a state of even-sized tiny grains of earth.
from the Guardian
You can also have too fine a tilth. My soil is great to work with and goes to a very fine tilth with ease. There's two problems with it though. Firstly it doesn't hold the water very long and, especially with new seeds, you need to keep them watered all the time.
The second problem is that the top of the soil cakes, so you need to water very carefully or it will run off and you also need to break it up when you can. The way round it is the same as when you have a heavy soil and that is to add loads of organic matter.
Coarse soil
The clumps mean that I cannot directly sow seeds- certainly not small ones like spring onions. I did try spring onions but not one has germinated after 4 weeks, and I blame the clumps.
from grow your own

As we are on heavy clay, getting the ideal tilth is almost impossible. If it's dug early and raked to a fine tilth, it 'caps' with rain or watering making a hard pan seeds can't penetrate. Dug later, there are heavy clods.
We get round this in two ways. First we start as much as possible in modules. We plant out when large enough through a deep mulch which keeps the soil fresh and moist. Works well for most things like brassicas, beans, peas etc.
For small seed like carrots and onions, we make a drill, water it well, and then cover the seed with vermiculite or sand (vermiculite is better). Water with a fine spray if dry and the seeds will come through even in heavy soil.
But if you have a nice sandy or loamy soil, then you are lucky and shouldn't need to bother with this, though I'd always recommend heavy mulches to improve soils further and reduce the need for watering.