The reason that discretionary income involves money that you can afford to lose because you make a choice.
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You clearly explained what discretionary income is money you can freely choose how to spend. This freedom makes people choose to invest buy things or save. However every choice has consequence: if you spend all your money on expensive habits now you might regret it later when you have no savings. We need to find balance between enjoying life now and investing for future. For example putting some money into Bitcoin is choice that might lead to growth unlike just saving cash which loses value over time. Ultimately these small choices control our financial future.
People work so hard just to earn money, that is, investing money is the center or purpose of physical and mental labor, but there is only one thing, that is to earn money. Just as we have to work physically and mentally to earn money, we should also think about how we are spending money. We have money now, so if we continue to spend money without planning for the future, then when my income will not be the same, then we may face financial problems. It is always wise to think about the future, a person is earning a good amount of money now, so he is living a good life, but he is not planning for the future, so his future is uncertain, but in addition to earning a good amount of money now, he is saving some money for the future and investing some money, but they are showing wisdom and they will have a good support in the future that even if a financial crisis arises, they will be able to deal with that crisis with the money saved, as well as use their previous investments for any future need.