Cabbages - why in a seed tray??

Your title says 'tray'' but your text says 'bed'.

Anyway, surely it's just a suggestion. You sow seeds in a nice well tilled bed or in compost in a tray first, then transplant the best ones to their final destination. Same result if you sow them in a pot. Or you could sow in their final destination and thin them out, leaving the good ones in situ.

It's your choice.

Also you can sow them away from slugs and snails and transplant when they are a bit bigger. Anything I direct sow has a tendency to get decimated by slugs and snails.

I'm with the put them out after a cash crop into a well prepared space school of thought.

The reason that I put my cabbage seeds into a tray is that I know where they are so they don't get weeded out.  It also saves the slugs, snails, sparrows and assorted wild life getting to grips and eating seedlings as slugs and snails are no respecters of netting.

Likewise I prick out the best ones into little pots of compost and plant out when they have good roots and a good top on them. That way I can see where I'm going with things. It also gives me two weeks notice to put lime on the brassica bed and well as fertiliser of choice and extra compost if required.

Seems to be the way we all do things up here. Could also be pigeon proofing (well we do have both wild and racing pigeons on our site and though the racing pigeons aren't supposed to feed away, there will always be one ...)