Function refers to the primary use of the object. In electric guitars the primary use is to allow the user to produce an amplified guitar sound. Obviously we are way beyond the basic needs of function in our days and times, and to consider that a product responds in a functional way we need to consider function in a broader sense.
We need to consider the product's goal, but also the persons that will use it, and how they intend to use it. This refers to a product being adapted to its target audience, and to what this audience intends to do with it. Guitars can be adapted to the genre of music for which it is intended, in its sound, its aesthetics and its ergonomics.
Sound and ergonomics are easy to remember when considering the guitar function, but if we consider that the guitar is also a stage instrument, and it's integrated in a carefully planned visual spectacle its aesthetics are also important.
In terms of ergonomics we can refer to the concept of usability, and why we need to make sure the design can be used with ease. For example, have the right neck profile, the right action and setup and the right pickup configuration for the specific user needs. We also need to consider to the best of our possibilities the impacts that an instrument can have on the body of a professional musician. Linda Manzer wedge guitar is a good example of considerations.
Complementing the usability of a product we have its efficiency, the reasons why the product functions correctly and with ease. The efficiency of a product has to be measured against the desires and ambitions of the end user. For example the stratocaster volume knob does its function on controlling the volume of the instrument and for some players it is located at the best position, making it extremely efficient. But for other players it interferes with their playing techniques, so much so that it makes the instrument almost unplayable for them!
The functional dimension of a guitar seems to be a very resolved issue if we keep to the standard iconic models, but if that is all we bring to the table, more of the same designs from half a century ago we need to consider why do it in the first place. Guitars, and electric guitars in particular have evolved to serve a very diverse musical landscape and they need to function in each specific context.