Both are needed, but for an athlete watts is by far the more useful feedback for training, and following the intended stimulus of workouts.

As a workout, "bike 30 km" is vague and meaningless. "bike 30km in 1h" is better, but not specific to the athlete: some have better cardio than others. Most serious cardio workouts are given in zones: "bike 1h at 60-65% of your functional threshold power" [1] – this particular workout would also be known as "zone 2 training session".

(1) https://www.trainerroad.com/blog/what-ftp-really-means-to-cy...