Your mention of problems seeing the service pack \go on from disk would be the thought there. If the disk turns out to be a bad copy while not the common but still possible occurence a separate download would simply be an alternate for seeing the same end result.
With some optical drives they have a tendency to be media fussy as well. While the full install disk wouldn't see any problem the media used for the SP might be that exception. The same goes for the blank media used to see the iso image burned to disk or you may simply have a need to download another copy of the iso if the first was incomplete for some reason.
With the latest AVG 8.0 out the first download saw "files missing" during the installation and closed. I subsequently downloaded several more for both versions as well as onto the external drive in case of another incomplete to see working results the next time around.
One little oops while downloading can goof things up for any program or update. This is one reason I prefer more then one download source where if one copy is incomplete dump that and try the next. If you ended up with a bad iso for some reason a fresh copy or alternate would be the idea there for simply seeing working results.