Exactly when you earn more through different streams of income, the standard of living increases alongside that's to say depending on one source of income can't create a balance for ones financial stability, but sometimes some person's savings plan can't improve their standard of living which is why you find them taking koans and being in debt. But having a multi streams income can ensbke one achieve that.
The rich don't get richer by using only one stream of income not minding how huge their income is, multiple streams of income reduce the risks of relying on a particular source and unfortunately that particular source of income becoming less effective to give the same return.
For people who work in jobs that one day will seek retirement, and won't work no more will have to settle into another source of generating income, for efficiencies and space to grow the source, it will be best to proceed a new source when the income still flows directly than time of retirement.