Why do we buy compost?

Same as TMTT. My clay soil is too heavy and can dry to a brick. I try to reduce my compost use by single sowing in modules and planting out when quite small, with fleece etc. I have also reduced "wasteful" sowings by calculating how many plants I can realistically grow on.
Most of my seeds are started on heat - clay soil drys like a brick. I once used mole hill earth for my chilli plants in pots - OMG! I couldn't believe how heavy they were. Was a complete disaster.

I use the compost in my bins for cuttingshrubs and dahlia tubers that I start in pots. Putting an inch or so of bought compost on top. I try my best to not put weed seeds in my compost but its always riddled with them.