When you use a wallet that connects to someone else's server, you're trusting that server to tell you the truth about your balance, your transactions, and the state of the network. Most of the time it does. Running your own node removes that "most of the time."
A full node downloads and independently verifies every block against Bitcoin's consensus rules. Nobody can hand you a fake balance, censor your transaction, or tell you a rule changed when it didn't — because your node checks for itself.
It also protects your privacy. Wallets that rely on outside servers typically reveal which addresses belong to you just by asking about them. Your own node answers those queries locally, so that information never leaves your house.
None of this requires you to be an expert. It requires infrastructure that does the verification for you — which is exactly what a Start9 server running a full node is for.