The values 0 - 16 are there for backwards compatibility with Workshop 1.x. If you set the teacher assessment's weight to 8 while keeping the other two at 1, the teacher's grade will be 8-times more significant in calculations.
doesn't seem to have any effect
You should see the effect of the weight once your workshop is in the Grading evaluation phase and you click the "Recalculate grades" button. If a submission received grades A and B from peers, and the grade C from the teacher, and the teacher's assessment has weight set to 8, then the final grade should be (A + B + 8 * C) / 10.
See http://en.wikipedia.org/wiki/Weighted_arithmetic_mean for full explanation.
Also note, the grading evaluation method Comparison with the best assessment takes the weight into account, too. In your case, the teacher's assessment will be always picked as the referential ("best") one. See https://docs.moodle.org/en/Using_Workshop#Grade_for_assessment for details.