Non-mining nodes do help secure the network if the purpose is self-hosting a wallet.

There are 3 main power sources in Bitcoin ecosystem, they exist in a balance-of-power arrangement (similar to 3 branches of govt):

By "running your own node" (and hosting your wallet therein) you are keeping check on Miners / Developers. Miners can't come up with crazy ideas that widely used wallets will ignore....similarly Developers can't come up with crazy ideas if either Miners or Wallets refuse to install their code....