It will be usefull because sometimes wee have lots of berries on
the berry bag that literally are useless, they will have more uses
on the berry bag like for example selling them or delivering at the
moomo ranch
There's a good reason for why you can't put berries from the garden
into your item bag. After you purchase the second garden, you can
get hundreds of berries. And berries sell to the item shop for 20
each, so having 100 berries would sell for 2,000 Pokedollars, which
is a lot. You'd be getting rich in a few hours, thereby probably
damaging the economy.
As for uses of berries, you can sell them at the Berry Market, bake
into food to sell, and sometimes there might be a mission for
certain berries on the billboard. They aren't completely useless,
they might just be sitting there for a while.