You increase your pension fund assets and improve your financial security in old age. Depending on the amount you purchase, you can thus increase your monthly pension. For people who have a gap in their pension cover and are looking for security, this can make sense in order to maintain their standard of living in old age.
You save on tax. If you voluntarily pay an amount into your pension fund, you can generally deduct this from your taxable income. This reduces the amount of tax you pay in the year you made the payment. You also benefit from the often higher rate of interest earned on your pension fund assets compared with a normal bank account.
If you make a voluntary purchase into the pension fund, you can still withdraw some or all of your pension fund assets as a lump sum at a later date, though this will depend on the provisions applied by your pension fund, which you should definitely check before any purchase.