What can you do that a fresh-out-of-college teacher can't?
I can plan lessons that include activities tailored for the needs of specific students. Training teachers find this hard and tend to plan for the median.
I can plan lessons more quickly than a training teacher, thus liberating time for more tailoring. I can use technology to differentiate delivery and to save time.
My feedback to each student is more accurately geared to that student's 'zone of proximal development'. I can set targets that mean something to each student.
I can 'reflect in action' in the classroom. I can read the situation in the whole class, and I can understand the logic behind mistakes that individual students make, and suggest alternative approaches.
How can you show that you do your job better?
In the institution where I teach we have QA observations and peer observations in place. That gives me some kind of benchmark.
Education is fuzzy, hard to measure, hard to standardise and quite hard work if you do it properly. As are many human occupations.