If you can max out your 401k or max out your IRA and then save an additional 20%+ of your after-tax, after-retirement contribution, good things really start to happen. If one is looking for earlier financial independence, such as retiring in their 40s or early 50s, it may be a good idea to skew towards more after-tax savings and investments given one has to wait until 59.5 to withdraw from their 401k or IRA penalty-free.
Portfolio income can come from multiple sources – interest/bond coupons, stock dividends, financial strategies including derivatives and capital growth.  Each offers some cash flow and some also offer potential capital gain with some risk from liquidity and volatile prices.  In this low-yield environment, many investors rely too heavily on cash flows and to pursue this are venturing further into riskier areas than they’d normally consider.