You can influence the quality of the future bonsai right from the start. Subsequent correction is difficult to achieve, especially when it comes to the roots. When growing bonsai from young plants, a good root approach (Nebari) is not a problem.
If the original plant for the bonsai tree is not too old, the trunk can be shaped very well according to your own wishes. In the case of purchased prebonsai (= bonsai blank) or even bonsai, the shape of the trunk can usually no longer be changed.
Growing your own bonsai from "bonsai seeds" or young plants is a good opportunity for beginners to gain experience without having to use high-quality bonsai raw materials. Many young plants can be used to test how the respective tree species react to different bonsai shaping techniques. If you feel safe then you can always buy a good bonsai or prebonsai.
Another important advantage of growing your own bonsai is: You only need a little money to get started. Young plants with naked roots from native tree species can be obtained in forest tree nurseries for less than 1 Euro, pre-styled bonsai young plants cost only a few euros.