I am talking about things like the cradles that Salsa and Revelate make. The most obvious reason seems to be to keep the drybag away from cable and cable housings. However, on my particular bicycle, I have wide handlebars (Nitto Albatross) with full cable housing, and it doesn't seem like that's an issue for me. I've done test rides with a drybag strapped directly to my bars, and it seems fine. However, before I head off on a tour, I'm wondering if there any other reasons I would want a cradle that I'm not thinking of.
I have a Wildcat Lion front handlebar harness which I got after previously using straps to hold an Alpkit dry bag to the bars.
Easier to attach the dry bag. If you've tried to balance the bike in the middle of nowhere in the rain after packing up you'll maybe know what I mean!
More stable (the Lion straps anchor to the fork crown as well, not sure if that helps). So doesn't rub at the bar/strap interface.
Holds the bag away from the cables and fork crown (which you already noted).
I think it's worth it, but they're not cheap so it's maybe worth seeing if you're going to go more than once before spending the cash.
Having now toured for two weeks using a dry bag strapped directly to my own handlebars, I can now sort of answer this question. I still haven't toured with a cradle, so I can't give a full side-by side comparison, but I can say that I spent two weeks riding without one and I didn't particularly miss having a crade. All of the comments by u02sgb in their answer seem potentially valid – I can imagine maybe it being more stable or easier to attach (edit: mostly easier to attach, stability was fine) – but those seem more nice-to-have than a must-have.
The main reason I can think of where it might become more of a must-have is if you were going to be on a multi-month tour, you might want one to prevent bag wear; I noticed some wear to my drybag where the straps went. Not a lot, the bag is far from worn out after two weeks, but it probably would wear out faster than a drybag would ordinarily. One could probably also mitigate that by getting a drybag that has reinforced tiedown points, or some other scheme to protect the bag from the strap (a wider strap, or a strap through a bit of used firehose perhaps).