Well, imagine this:
I'm starting a new web site for a customer.
I build up a joomla site and pick up the needed template.
Inside the template i have almost every thing to work with it.
When I'm assigning modules, I must use some kidn of template so I can check where every module is. I have two ways to make this task:
1. the long way:
I open navigator and get to
www.icetheme.com/
Click on "template demos"
Click on drop down and select my template
Click on "layout" menu entry
Scroll down to get the screenshot
Download that image to my working directory
Print that image and put it next to my keyboard while assigning modules positions
2. The desired way:
I have my template already downloaded, so I double click PDF layout document and print it
Now imagine I'm working out of the office, during a plane or train travel, or even in the beach house I rented 2 weeks ago. Do I really need a 24 hours mobile internet connection just to check modules positions? It would be crazy!
And if my internet connection fails for some unknown reason, I have no way to get a simple layout image with all named modules.
Let's imagine a little more: you get a new ikea furniture

and, of course, it's a do-it-yourself package, so you grab a screwer and a beer and start rebuilding your brand new "Ammerö" table. Then, you realise that there are no instructions papers, and you have to go online inside ikea's web site, search, download and print that paper, wich is an useless and time-spending task. Wouldn't it be better if that paper was already inside the package, so you have all needs together from the first moment?
Layout image (for me at least) is a useful instructions paper to build up a site with a downloaded IT template. I'm not asking for a hard work fot you, I think it's not too complicated, and with a little job on your side, things can be enormously simple and quick for us the developpers.
I hope you can understandmy needs. Thanks for asking

P.S. And sorry for my poor english and expressions.