I find that practicing some finer movements in slow practice can be very challenging with lunges. There’s a point in a lunge where you lose all control until you stop lunging. This makes it very hard execute techniques that occur close to the start of your lunge. Doing certain techniques at speed is actually much easier.
For these techniques it’s valuable to know how to pass well; you can maintain complete control over your forward movement doing a slow pass. And passing lunges can vary in the distance covered: you can do a passing lunge that covers the same amount of distance as a lunge.
I often use passing lunges when demonstrating techniques in class so that my students can track the movement of my upper body and sword better. There are also very good reasons to be able strike on the pass at different measures with swiftness.