Why do we buy compost?

Thanks for your replies.
As you may remember I bought a garage load of Coir last year (being sold off in Morries). I thought I'd be able to start seeds in that but its not working for me. Poor germination and those that do germinate are weak. For the first time in living memory I have NO tomato plants.
Currently, I'm mixing the coir 50/50 with a bag of compost which is giving better results but I've lost confidence in the coir. It may end up as top dressing on beds as I don't want to risk sowing seeds, or planting tomatoes in it.

100% coir can be tricky to grow in. It's easy to overwater because the surface dries out quite quickly even though just underneath it's still very wet.

It also doesn't contain much nitrogen, which is probably the most important nutrient for plant growth. Seedlings shouldn't need extra nitrogen until they've reached the true leaf stage, but growing in 100% coir after that point will require frequent fertilising.