if u have 2 or 3 pipelines in your mapping,which pipeline
will load first?
so taget load plan option will give u the availability of
which pipeline will load first and then which one is second
u can decide based on requirement or your logic.
if u doesn't give tagetload planing option,informatica server
will load the pipelines one by one but how it will take we
don't know.

As Per My Knowledge The Taget Load planing Is Most 
Important Event While Loading The Target Generraly One 
source Qualifier  In Our Mapping then we No need to Think 
For This I Mean Seriously, When It Has Multipul Source 
Qualifiers Then The Problem Is Arraised,  Then We 
Designated Which Pipeline Is First,Then Which One Is 
Next,If We Did Not Specified Proper Method To Informatica 
Server Loads Targets But It Doesn't Tells Us
Proper Order So Always We Specified   Order To Load The 
Target.