Why do we buy compost?

I sow most seeds in or prick out into modules, and soil just isn't a very good growing medium in containers. You can have it as part of a mix, but 100% soil is too heavy, compacts too much and holds on to too much moisture. Now I've moved away from growing a lot of things in containers, I buy much less compost. I only really use it as part of a seed sowing mix, as the castings my wormery produces are too fudgy to use on their own and my homemade compost is full of miscellaneous seeds. The only things I sow direct are peas, carrots, parsnips and certain flowers.

Growing in modules gives me more flexibility - I can start things earlier and keep them indoors or in the greenhouse, they are less likely to be damaged by slugs etc., I can have plants at an already good size for when bed space opens up, it is easier to transplant them without damaging roots, you have more control over germination conditions, there is less thinning out...

If I had a good supply of leaf mould I probably wouldn't buy any compost at all, but we don't have any big deciduous trees in our garden and any leaves are left where they drop in the borders.