The purpose of the Security Buffer is to prevent you from retiring with less money than you’ve invested. On top of investing in a risk-averse fund, you have an extra layer of protection against large scale downturns, like a global recession.
It’s likely that you won’t need the protection of the buffer since the type of investing you’re doing has always turned out profitable in the long run. However, it’s nice to know that you’re relying on something more than luck when your retirement date comes along. If your investments have made an overall loss, the buffer will reimburse you.