This could apply to a number of plants but for the purposes of this article, its about Leeks.
Leeks need a long growing season, I usually make my first sowing indoors in January to annoy Kristen Followed by further sowings from Feb to March to extend the cropping season.
Now you wouldn't want to plant them out in their final growing position as soon as they were big enough as that would occupy a large amount of the plot for a long time. Land that could be producing something else in the meantime.
Thats where the "holding bed" comes into play, whether you've grown from seed or bought some trays in, it pays to get them in the ground somewhere.
Doesn't have to be a big space, this year I put mine in between the small saladings.
Anywhere, once they have been hardened off, just to let them get their roots into proper soil.
From the same sowing, the one on the left was in a holding bed, while the one on the right got left in a pot.
Note not only the size, but the difference in root growth.
You don't need to separate the roots or thin them out when putting into a holding bed, just slip them out of the tray and let them find the soil themselves.
That way, the only root disturbance comes when you transplant into the final growing position, less growth check.