I'm not sure if this will answer your question, but I'll give it a try.
You can scale your objects using either the "Scale" fields, or the "Dimensions" fields:
While you can give your object exact dimensions using the "Dimensions" fields, you can alternatively use the "Scale" fields to, say, reduce or increase your object's size by a given %.
Let's say I want to stretch Suzanne's head (whose dimensions on x,y,z are: 2.734, 1.703 and 1.969 respectively) a 25% on the z-axis. What I'll do is grab the z field inside of "Scale" and change it from 1.00 to 1.25:
X and Y dimensions remained the same, but the Z dimension increased by a 25% with respect its old value. Now I simply go to Object > Apply > Scale to tell Blender: "Hey buddy, these dimensions will be considered from now on as the default dimensions upon which I'll be working on. Take them as the REFERENCE for subsequent operations":
How would I increase the Z-dimension by a 25% without the "Scale" field? I would need to type inside of the Z-field inside of "Dimensions" the following: 1.969+0.25*1.969
So, the "Scale" fields are there for pure mathematical convenience, as far as I know.