In the good old days, before autothrottles, the pilot flying would advance the power to begin the take-off, and call something like "Set Thrust".
The pilot not flying would then fine tune the thrust to the calculated take-off setting, which takes a few seconds. This allowed the pilot flying to concentrate on steering the aircraft down the centre line of the runway.
More tradition than anything else on an autothrottle equipped aircraft - a manual backup, if you will.