ITER was planned in part through early results from JET in the 90s - but they more recently replaced the inner wall of JET to match what ITER will use, which was a good choice according to models but this gives empirical confirmation that it's possible to sustain the plasma using the new wall material.

All new fusion plants are a risk, that's why they're experimental. I suspect they were reasonably confident they could make ITER work and that's why they started building, but this will give them confirmation that the material choice was a good one and will also show them in advance some of the operational obstacles and possible solutions.

In other words, they knew "where they were going to go" but this gives them more confidence they were correct in deciding the direction, and will speed up their learning curve setting up the machine once built. Even if it is built and something fundamentally doesn't work about the concept, that will still be useful scientific knowledge, even if it is disappointing.