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.