Domestic flights are mainly used for business, with the train being a simpler and often (but not always) cheaper alternative for other travel. The boom of budget airlines and increased competition has made some flight prices competitive with trains to some major cities. However make sure that you get to the right destination. Low-cost airlines (in particular Ryanair) are known for naming small airports in the middle of nowhere by cities 100 km away (e.g. "Frankfurt-Hahn" is actually in Hahn, over two hours away by bus from Frankfurt city).