In my experience, a backpack is way more practial (your hands are free), and you can pack quite a lot of stuff in a 80-90 Liters backpack (if you know how to pack to save space).
For day-trips (e.g. hiking) you'd need a smaller, ca. 30 liters backpack (you can carry it on one shoulder even while carrying the big pack), and you can carry wallet, passport, cellphone, etc. in a small always-on-you bag.
In my opinion, 'a 80-90 l backpack' and 'practical' in one sentence sounds odd... and a 30 l one for day trips?? Last years, I've always travelled with a backpack of 27 l, and that was big enough!
Well, the size of the backpack depends on the person's size (literally, and because a bigger person has bigger clothes). But, yes, a "travelling" backpack (the one used instead of a big suitcase), for a longer trip abroad, will end up being somewhere in the 70-90 L range (again, depending on the person, season, purpose of travel, etc.).
A backpack of 30 L is not for strolling around the city, of course, but for daytrips, hikes and the like. Of course it can be smaller, but I always prefer to have some room left for things I might buy along the way - the pack fills up quickly if you put in a bottle, some food/snack, various smaller personal items, a jacket...
And I'd call any backpack 'practical' in comparison with a suitcase or a shoulder bag.