Online storage services are useful for continual syncing, which using an external drive would be inconvenient for (if that drive was stored off site). Glacier is for files that you aren't likely to access again for months-or-years-on-end, unless there's an emergency or you're doing something bespoke, like making a family photo album and need to go back through all your files again.

So filling up an external drive and keeping it off-site and using Glacier is not as much apples and oranges.