Help me understand how this works in a real world situation? If you immobilize with Might you are grappling and have to stay with the target. If you use any other attribute including Agility you can move freely.
How are you using agility to immobilize without grappling or at least using some kind of arm hold or something that would require you to stay on/with the target?
Might is the idea of a person physically grabbing and holding onto the person, and thus able to move them (notice the only way you CAN move a target that is immobile is via Might default Core [though I could be remembering wrong])
Agility is perhaps using something like a bolo, but in most cases a precise strike to a nerve or series of nerves to cause the body to seize or freeze up.
Lots of ways to describe it if you look for it.
In your games, the GM can always choose to allow other attributes to behave similar to how Might does with Immobile, especially if you describe it as you holding them.
In my games, I allow Might to access persistent damage for example.
Context means everything in Open Legend. Just because you use Might to invoke Immobilize doesn’t mean necessarily that you grapple someone. You could just as well throw a net over someone’s head. On the flipside, you could flavor immobilize via agility similarly to grappling someone, like for example with different types of wristlocks, and thus could apply the same “grappling” rules from the description, which you cited to agility. The rules just serve as guidelines and it just so happens that most often might will be depicted as grappling and that’s why that clause was added, but it doesn’t have to mean that the clause is exclusive to might and always applies on immobilize with Might.