One can trade more rationally and make less emotional decisions when his income is steady. During winning runs, many novices become overconfident, but the market could change at any time. Long-term success requires dedication, planning, and ongoing education. The best strategy is to balance trading with a reliable source of income.
This advice you put forward is very suitable for every trader and not only for beginners, although beginners should follow such things more so that they can have many wins through the trades they make. Because sudden market changes are quite valuable lessons for all traders, so every trader should not avoid a more mature strategy and planning for themselves continuously without feeling bored and tired.
Let's say we only have money that can be traded and that's the last money we have. The risk in trading is very big, so when we have to feel the loss and lose our last money, then it becomes a problem, while we have no income. So having a stable income is more advisable, because even though we might end up losing in trading, we still have other income. It might be different if it's a professional trader, but what I know is that someone who is successful will have many sources of income.