Why do we buy compost?

I have started moving away from twig and plastic compost, ie shop purchased.

I have two dustbins that contain spent compost and vermiculite, I feed it from time to time with riddled compost from my main compost bins and a little liquid feed, worked fine last year and so good so far this.

As JJB says, the cost is getting silly now so I shall hold off buying for as long as possible.