What does the Intervals.icu connector add?

Intervals.icu is an aggregator, so it can carry data Trailwise cannot reach directly, including data that originates on a Garmin. It syncs activities and wellness, and can fill in a blank FTP from your cycling settings (it never overwrites a value you set). It also has a plan direction setting: leave it off, push your Trailwise plan out to Intervals (and on to a watch), or import a human coach's Intervals plan as your active plan. The two directions are mutually exclusive and both opt-in.