Hi ACP - It is an important phase of flight when you change from flight level to altitude
(1013 to QNH) whether climbing or ascending. You can instruct a Flight Management Computer (FMC) to do most things, but changing the Altimeter setting is a big part of the relevant manual checks while leaving or approaching an airport. Important for traffic seperation while climbing, and important for height above ground information while decending.
Hope this helps - if not, shout and ill have another go!!