It depends on how you define "secure the network."

Your own node relays transactions, manages a mempool, allows you to privately access and verify chainstate, and validate your own transactions, among other things. It's a pre-requisite to route lightning payments and help build out a credible decentralized medium of exchange functionality for btc, which accelerates its monetization. Collectively, these are all important for the socio-technical ecosystem that btc functions in and is composed of.

Btc is more than just a technical specification. Not understanding that is to miss most of the important things about it.