Don’t have to be washed between uses. Seeds deal with all sorts of things in the soil outsie.
No need to thin plants – if you sow direct you have to thin out.
No root disturbance, or minimal anyway if you have a handy tool to pop them out.
Can reuse them time and time again.
Can start things indoors earlier
Allows you to start many plants at once.
Less growth disturbance when planting out.

Fill with riddled compost if possible. Don’t use chunky compost!
Fill, scrap over with hands and then bang trays down.
I like to firm the soil with fingers before sowing seeds.
I think the more compost compost makes it easier to get the roots out without damage. A firmer soil plug is easier to handle. I like to cover the seeds with more compost.
I do one seed per module rather than having to disturb roots and thin them out.
Some people say put a few seeds in for things like rocket, lettuce (non-hearting) and other things that don’t mind being bunched together.

Mind the slugs and snails underneath – they’ll love the hiding spaces module trays have.

They’re great for starting off larger numbers of plants in the safety of the greenhouse.

Make sure you label them! Especially if you’re doing multiple varieties in one tray.