You probably should look up the definition of Enum if you want a deeper explanation on this.
An Enumeration is a set of VALUES logically related to each other. Since you are late-binding, the compiler doesn't know what value to assign to acAlignmentBottomCenter (13 in this case). The acAlignmentBottomCenter variable is really a shared constant with the value of 13. The AcAlignment Enum is just a nice way of saying...I have this group of unique values which I want to assign names so that others will know what the values are for.
So, not only do you need to declare the constant acAlignmentBottomCenter, but you also need to assign its value of 13, because this is ultimately what AutoCAD 'sees' and it knows that 13 = acAlignmentBottomCenter.
This means that if you know the value of the Enum Member you want, you don't really to define it at all, you could just use 13. But acAlignmentBottomCenter doesn't equal anything if it is not defined.
Also note that Enums can be any of the following: Byte, Integer, Long, SByte, Short, UInteger, ULong, or UShort.