Questions regarding portmaster's man page

[Christer Solskogen]

Hi!

The man page for portmaster say this:
 Alternatively you could use portmaster -a -f -D to do an ``in place''
 update of your ports.  If that process is interrupted for any reason you
 can use portmaster -a -f -D -R to avoid rebuilding ports already rebuilt
 on previous runs.  However the first method (delete everything and rein-
 stall) is preferred.

I'm wondering why the first method is preferred.

-- 
chs