What's the use of the logOnly option in ngrx DevTools?

If you scroll down a little bit on that link you sent you will see:

logOnly: boolean - connect to the Devtools Extension in log-only mode. Default is false which enables all extension features.

With a link to the extensions features.

Based on this we can assume that setting logOnly to true will switch the following redux-devtools-extension features off:

const composeEnhancers = composeWithDevTools({
    features: {
        pause: true, // start/pause recording of dispatched actions
        lock: true, // lock/unlock dispatching actions and side effects    
        persist: true, // persist states on page reloading
        export: true, // export history of actions in a file
        import: 'custom', // import history of actions from a file
        jump: true, // jump back and forth (time travelling)
        skip: true, // skip (cancel) actions
        reorder: true, // drag and drop actions in the history list 
        dispatch: true, // dispatch custom actions or action creators
        test: true // generate tests for the selected actions
    },
});

This is ideal for production environments as you may not need or want these primarily dev-focused features running in your live application.