Because tax on registered income is rarely flat across your life. Pulling RRSP money in low-income years, before CPP, OAS, and forced RRIF minimums arrive, often means paying at a lower rate now instead of a higher one later, when several income sources stack in the same year.
Picture a couple who retire in their early sixties before starting CPP or OAS. Their income may be low for a few years. Leave the RRSP untouched and it keeps growing, and so does the eventual forced withdrawal. When CPP, OAS, and the RRIF minimum all switch on, they can have more taxable income than they ever earned working, with far less control over it.
Drawing modestly in those quiet years uses up lower-taxed room that would otherwise be wasted, and shrinks the balance that later drives those forced withdrawals. The cash you do not spend can move into a TFSA, where it grows and comes out tax-free, or a non-registered account. You are relocating the money to a friendlier tax address.
This is not a blanket rule. Pull out too much and you push yourself into a higher rate today for no reason. The smooth path is rarely the one the rules hand you by default; it is one you choose deliberately.