Some recipes use water in nut brittle and some don't, why?
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Mostly to make things easier for novice bakers. The water reduces the chance of scorching/burning the sugar for the caramel. An longtime baker will have enough experience to know about the risk of burning the sugar.
In a similar way, most consumer recipes will call for the use of a double boiler to make hollandaise sauce. That nearly eliminates the chance of curdling the egg yolks. However, in a professional kitchen, cooks put the yolks in saucepans and cook directly over the stove. It's much fast this way (time is crucial in a commercial kitchen), but there's a higher chance of ending up with scrambled eggs.
There are warning signs when the yolk mass is reaching a temperature that near the curdling threshold (i.e., a certain amount of steam coming from the yolks), but it takes some experience to recognize this, plus the knowledge of what to do if you see the signs (pull back from the heat source, ladle in some water to thin/take down the temperature).
A lot of this type of knowledge is not optimally communicated in the written word; visual demonstrations are more helpful, and live in-person hands-on experience is even more so.
So, some recipe authors try to keep things simple because they don't know the skill level of their readership. Their recipes are written in a manner so novice cooks have the higher chance of success. An experienced home cook or professional chef often ignore some of these beginner tips.