An apprenticeship qualifies you for an occupation. 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.