Well, for one, it surely has clearance for 25c tires, so there's that. I have done 23c on gravel - but! - with modern, Vittoria Open Corsa Evo SCs - and they didn't hold up too well. Of course, they aren't designed for that kind of abuse, but these old-school Contis? They look just right for that kind of stuff. The center ridge is almost 2mm taller than the rest of the tread! Plenty of rubber there.
Plus, the wheelset is lower on the spiffy scale than anything I currently have. And let's face it, the bike was a super deal at $650 (spares included), so it's the least expensive of anything I own. Kinda makes it the default, do anything-go anywhere bike.
Also, it's got a freewheel with better gearing than anything else I own, too.
Lastly, it's been waiting since December of 1979 to be taken out and given a good caning. I'm here to fix that
DD