Why do we buy compost?

I buy 120ltr of compost that's used for toms, cucs and any other seeds I start of in the house and are staying indoors until its time for them to go out.
It's mostly because the compost I make always has slugs and other seedlings popping up. After that I mix store bought/my compost together for potting on.

Like TTT I also grow in modules/newspaper pots I don't have a lot of space and growing that way means less gaps in the beds.