Is it necessary to add prefixes of halogens (more than 1) to the iupac name alphabetically? If I draw structure of 2-bromo-4-chloropentane and 
4-chloro-2-bromopentane I get same result, when I try to draw structure. So why is it necessary to name prefixes alphabetically. Can we write iupac name not alphabetically.
With the preferred IUPAC name (PIN), there is exactly one way that you expect to find a compound written.

Assume you want to buy the compound from the chemical supplier of your choice but don’t have a CAS number at hand. What do you do? You search for the compound in the catalogue. In the old days, these were always paper books that you could use as a weapon but nowadays electronic catalogues exist to. Still, if you’re looking for 1-bromo-3-chloropentane but it is entered into the catalogue as 3-chloro-1-bromopentane, you’re going to have a very hard time searching for it.

There are also other reasons why having one defined standard way of naming compounds is strongly preferred over random but clear.