I’ll try to keep this short.
When you include value as a facet of your expressions, you’re referring to the information you’d keyframe in whatever property your expression exists. Take the Position property, for example. If we want the ability to drag our layer around after we’ve added an expression, we’ll use value. (The alternate solution is using property-specific objects. transform.position, for example. value is just easier to remember. It’s a catch-all.)
When we don’t have value (or the property-specific objects) in our expression, the information in those attributes are ignored. Let’s look at a boring example– using an expression in the Position property of a layer. Here’s our expression:
Yeah. That’s it. Not very practical, but it illustrates the point. “What point?” you ask. If the above expression is in the Position property of a layer, that layer will always be at [100,100] in your comp (unless you go and do something crazy like parent it to a layer). No matter how much you try to drag your layer around in the comp window, or drag the Position numbers in the timeline, that layer won’t move. Let’s amend this expression with value.
Amazing. Now, the numbers in the Position x and y properties have an effect on where the layer is. If we punch 640 and 480 into the layer’s x and y (respectively), we get a layer at [740, 580]. (….because we added 100 to the x and 100 to the y! Pay attention, for crepe’s sake.)
Go add value, you filthy person.