The use of Budgets tool is to set yourself a spendings threshold that you don't want to pass over, for a certain category. It is used just to track your current amount of spendings on that category compared to your set budget, in a more visual way.
As for the Cashflow, you can go there and add future expenses on the fly. Just by clicking on the box you want (category / month), you add a future expense and that will project itself on the cashflow and you will be able to see how that affects your future wallet balance.
I hope this gives an idea on what are your possibilities regarding cashflow projections, and use of the budgets tool.
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