we are bestowed a fixed number of Breaths by HIM...and these breaths come at their appropriate time one by one..and when its all finished..we depart...Still its impossible for anyone to ask..why one breath at a time..since they all come form HIM..why not all at one GO...
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-sikhi-sikhism/32036-why-a-daily-hukamnama.html

Sri Guru Granth Sahib Ji is JEEVAN JAACH..Method of Living our LIFE on a breath by breath basis..so that each breath is blessed by HIM....Sri Guru Granth Sahib Ji teaches us How to do THAT....and since we live life day by day...Hum aadmai haan EK Dammii..says Nanak..the Sri Guru Granth Sahib Ji hukmnamah on a Daily Basis is a most convenient way to assimilate the Jeevan jaach into our Life. We have for convenience sake divided up our Lives on a Daily ( 8 hours work..8 hours sleep etc..) 24 hour ccycle....and so similarly we have set up a Daily hukmanmah basis so that EACH DAY we take DIRECTION form HIM on how best to live our LIFE so that ultimately at the End of the "DAY" (for some a few minutes after birth..to others a ripe old age of 100++)..we RETURN TO BASE blessed by HIM..to be at His Lotus feet.