Until the early 90s, it was commonly assumed that your withdrawals from a portfolio could be about the same as the average market return, and if you stayed below the market average then your portfolio would last forever.
But this strategy failed to account for the fact that the market for you, when you happen to retire, is not the same as the average market. The market conditions that apply when you begin taking distributions strongly influence portfolio outcomes.
If you happen to retire into a period of poor market performance, this can have negative effect on portfolio longevity (meaning: you run out of money). This phenomenon is known as sequence of returns risk. Markets are volatile, so no two 30-year retirement periods are exactly the same, and you must account for this to determine your safe spending level.