Weightage value is a relative measurement of impact any KR or objective alignment is going to have on the overall objective progress. Thus if there is only one KR with weightage value 10, it is going to have the same impact even if its weightage is updated to anything more than 0.
0 is a special case of weightage - when set, it doesn't impact the objective progress at all.
Formula for calculating the impact is same as that of weighted average. For example, if there are 4 key results to an objective (and also this objective does not receive a contribution from any other objective) with following weightage values - 3, 5, 7, 10. When the first KR with weightage 3 is 100% complete it will impact parent objective progress by = 3*100/(3+5+7+10) = 12% whereas the fourth KR with 10 weightage will impact by = 10*100/(3+5+7+10) = 40%