Credit history is just a history of making payments on-time. You can build a history by making student loan payments, car payments, health care payments etc. Obviously it's only one factor in someone's credit worthiness, however it's not a bad metric. If you pay your bills on-time, it makes sense that a lender would value you higher/safer than someone who does not.

> the random timeframe in which the (still unnecessary) credit is free

Are you talking about the 0% APR period, or the monthly revolving cycle? The latter is absolutely predictable, and if you dislike manual payments you can always set up automatic ones.