why different swapping modes on executables?

[Steven King]

   TFM page for a.out defines two magic numbers 0410 & 0413 for executables,
 and describes the difference as:

        "A magic number of 0410 indicates thats the executable must
       be swapped through the private swapping store of the UNIX system,
       while the magic number of 0413 causes the system to attempt to
       page the text directly from the a.out file".

    While I think I understand what the difference is, I don't understand
 why the difference. Is there any advantage to one over the other? How
 does one, short of directly editing the binary, assign one or the other?
 ( the link editor doesn't offer any clues ) 
-- 
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