You can do what you like with them.
The chief reason for using a seed bed or trays is to save space/time and
to reduce the damage done by weeds.
40 plants in a plug tray take up much less space than when planted out.
If you allow 8 weeks from sowing to planting out for your Spring cabbage,
your summer cabbage and then your winter cabbage then that is 6 months of
the ground being taken up when other crops could be growing there.
Direct sowing with pelleted seed and the use of selective herbicides makes
direct sowing quite feasible, but with crops than can occupy the ground for
almost 12 months sowing to harvest then not so viable as direct drilling of
say lettuce. And I have in my day had to "single" many many acres of lettuce
that were drilled before the days of pelleted seed.

--
David Hill
Abacus Nurseries
www.abacus-nurseries.co.uk