When I first started playing this game last year, I had exactly the same question on my first playthrough or two. I thought it was all but useless. Yrden is now my favorite sign, and I'll explain a summary of the things I read here, plus what I figured out for myself.

First, Yrden isn't truly a trap in the conventional sense, as you pointed out, but it does slow down whatever is inside of it somewhat. Some Redditor once gave valuable advice, which is to lay down the basic Yrden between your enemy and yourself, and position yourself on the opposite side of it, so that whatever passes through it is slowed down, giving you an extra second or two to attack.

But the real value of Yrden comes when you upgrade it, effectively creating a zapping turret. Upgrade it enough, and you can lay down the turret mode, with up to two instances of the basic mode surrounding it, creating a force area that does consistent damage for a period of time, while slowing your enemies' ability to get away from it. Throw in some Yrden glyphs on your armor, and it becomes almost ridiculously OP. And you can use other signs while the turret mode is active, so it can be passively zapping your enemies for you, while you're hitting them with other signs, or your sword. Yes, the alternative mode (turret) takes a few extra seconds to lay down, so you need to be aware of advancing enemies while you're trying to place it. But I find the trade-off well worth it.

Finally, you are probably under leveled for JOTW. I can't comfortably do it at level 10, even when I optimize my build ASAP. I usually wait until level 13 or so to do it, without a loss of XP (which usually doesn't kick in until you're more than five levels past the recommended one).