But isn't the main purpose of personal finance to influence spending decisions? An ideal solution simply just answers the question "Can I afford to X?" I've no

But isn't the main purpose of personal finance to influence spending decisions? An ideal solution simply just answers the question "Can I afford to X?" I've not used any software that helps solve that problem in an elegant way, and maybe I'm just minimizing the problem, but it seems like every path to answering that question is simple arithmetic provided you know what other expenses you have in addition to X.