It's a conjecture based on a general premise that most designers have reasons for the things they do.How can you know that?
I mean, the causal explanation for why my computer has a keyboard is that someone put it there.
But they didn't put it there for no reason. There's a functional explanation of why my computer has a keyboard - it's a type of user input device. (I'm sure tech people can give a more elaborate account of the function. But that basic point seems fairly obvious, driven home by the fact that my other computer with a keyboard that doesn't work properly is gathering dust.)
Which decisions? I don't quite follow this question.And, how is their intention relevant to your decisions?