An apprenticeship qualifies you for an profession. For example if you have completed an apprenticeship as an inland sailor you can apply for a position to work as an inland sailor. If you do not have a qualification in this area this is not possible.
Without an apprenticeship you are left only with jobs as a labourer/unskilled worker (for example on building sites or in agriculture) where in most cases you will earn significantly less money.
Another argument: a completed apprenticeship protects you from unemployment. In addition, without an apprenticeship you are more likely to receive temporary contracts or you will only be allowed to work at specific times – for example in the summer when there is lots to do on building sites, in gastronomy or at the harvest.
Often trainees are taken on directly by the companies after their training. This means that you can continue working in the company. Many students also get to know companies during their studies or during an internship where they can find a job afterwards.