You might wonder why, and these are my reasons:
1) Developing your own site using your competitors site layout. This will save you lots of time thinking about how many menus to create, how deep the links go, how to attract and inform your prospects and such. A single run from a few competitors and you'll have a good gauge on how to develop your site.

2) Revamping and migrating your sites will be much more easier. I've just run it against one of my business website, and to my astonishments, my website has over 300 pages and I totally did not notice it at all ! I was about to revamp and migrate my website probably around December and thought it'll not take much time and effort since all contents are already there, but now that I know it has more than 300 pages, I better start doing my planning now.

3) Spying on your competitors' pricing. I have an e-commerce business and noticed recently my sales has dropped quite a bit. Ran VisualSiteMaps and only then did I realised not only my competitor is beating me in terms of pricing, their products have increased by at least 3 folds and product range is almost 5 times more than I do! It'll almost be impossible for me to know without VisualSiteMaps.

I can list a few more, I do, but my priority now is to kick my own butt and work harder to win back my customers.

So if you were like me being wishy-washy whether to get this tool or not, trust me, one run and you'll be impressed.