The #1 reason for contributing to an RRSP is to provide you with another source of retirement income. While contributing to your RRSP over your working years, you receive an immediate tax benefit as the contributions are tax deductible. Your retirement savings then grows, tax sheltered within the plan. The result is that when you retire, you have a nest egg to draw on to supplement your other sources of income.
If you have a spousal RRSP, the potential benefit is even greater. You have two plans to build up over your working years and two plans to draw on when retirement comes. In addition, income-splitting strategies during your retirement years minimize your collective tax bill.