You need discretionary income only when you are in the midst of some kind of financial crisis or.
You don't need discretionary income only when you're in financial crisis but you need to have discretionary income all time in order for you to be ready to solve whatever expenses that comes your way whether planned or not. This is the main reason why you're expected to set up various kind of backup funds to tackle such problems. You use your discretionary income to set up your emergency funds, reserve funds and float and all of them have their own purposes and the right time to use them. They serve as your discretionary income if you don't have any available at that moment apart from these three.
This is why you can use your reserve funds in a more flexible way by using it for either your wants or needs if your weekly income is delayed and replace it immediately, you are paid. If you are in any financial crisis, you should use your float or reserve funds. Only if it's a real emergency and your reserve funds have been exhausted in the process of solving the problem, you can tap into your emergency funds. And make sure to refill them for future crisis.
You also need discretionary income to set up your bitcoin investment and continue building it overtime, as a matter of fact you need discretionary income to invest in various kind of assets.