One of the conditions for CLB is that it has to be in a specific performance phases, GA phase
not included.
The FCOM seems contradictory because it shows graphics of FMAs with TOGA/SRS/GA TRK and CLB in blue below SRS. However when you accelerate it will be OP CLB, not CLB what you will get. Hence the need to select the first constraint.
It is an old subject of debate, but lately it has become popular. probably there was an incident for this reason… Problem is that now some people misinterpret it and is doing weird things, like setting 1,000 as the GA altitude because the MAP says "climb to 1,000 then turn right to ZZZ VOR climbing to 3000"
.