It's cheaper and faster. What's the hassle or inconvenience of a separate drive? If anything, Dropbox is less convenient because it requires installing an application to use it.
The only drawback is that you have to carry it around anywhere you want to access it. A modern external drive is the size of a phone though, so that's hardly a dealbreaker even if you do want to carry it around. Although frankly, cloud storage is much better for filesharing.
I guess it’s just about everyone’s slightly different scenarios. My typical example: I have all my photos on Dropbox. Some of them a decade old. Don’t need them taking up SSD space. Don’t need to carry them around. But once in a blue moon I want to see a decade old photo from my phone while I’m on the other side of planet. Same for my ebooks, older work files, tax returns, etc, etc.
The only drawback is that you have to carry it around anywhere you want to access it. A modern external drive is the size of a phone though, so that's hardly a dealbreaker even if you do want to carry it around. Although frankly, cloud storage is much better for filesharing.