In preceding Load you can make changes in your fields, for example:
a) Extract Month from a date: Month(date_field) as Month_field,
b) Make a flag: IF(field_value > 50, 1,0) as Flag_field_value,
c) Concat fields
d) Make Keys (autonumber)
e) Rename Fields
But First check always "QlikView manual":
"The preceding LOAD feature allows you to load a table in one pass, but still define several successive transformations. Basically, it is a LOAD statement that loads from the LOAD or SELECT statement below, without specifying a source qualifier such as From or Resident that you would normally do. You can stack any number of LOAD statements this way. The statement at the bottom will be evaluated first, then the statement above, and so on until the top statement has been evaluated.
You can achieve the same result using Resident, but in most cases a preceding LOAD will be faster.
Another advantage of preceding load is that you can keep a calculation in one place, and reuse it in LOAD statements placed above."
Miguel del Valle