Scouts sometimes have armies too. =)
Added after 1 minute
Or, for example, you need to quickly get a transfer hero to the main hero.
Either go through the whirlpool and lose the army (*sniff*) or swim like this... but it takes a long time.
There are places that can only be reached through a whirlpool.
Even if you're carrying "a couple" of populations to the main hero, you can still only lose the ones you don't mind losing.