70s hard rock.  SEO.  Whilst the connection you might be able to think of is that your web developer is wearing an "original" Rainbow tour t-shirt (despite having been about 4 when the band last performed live), there is much* that grizzled middle-aged men playing Smoke on the Water can tell us about SEO...

1. They're an authority group

Deep Purple have links through most of the great hard rock acts of the 70s - sharing members with Rainbow, Dio, Ozzy Osbourne, Whitesnake.  This is not a group on the peripherary of things - they are one of the authority groups.

Being an authority group means that a link from them has extra cachet (a high 'Rock Rank' or something like that...) Supporting Deep Purple were a band called The Crave.  Nope - me neither, but a link from Deep Purple to them now says that they're probably a band we in the audience should like - more so than any number of links from "Rock Band Directory UK" can.

See: Domain Trust and Authority

2. Great content

With a set list including Highway Star, Strange Kind of Woman, Wring that Neck, Hush and - naturally - Smoke on the Water this is definitely premium content.  Fresh content?  Some of the classic tracks don't get performed live very often, and whilst the last thing you want your favourite 70s rock group to say is "here's some of our new stuff", they do put a new spin on some tracks - and there's always the keyboard and guitar solos.  With back catalogue of 18 studio albums, Deep Purple have an enviable store of great content to dip into.

See: Generating Unique Content

3. Technical build

Deep Purple have had 14 members over the years, and this is line up #8 (according to the ever informative www.deep-purple.net). The core of this line up is pretty much your classic Deep Purple ca. 1969 - 73 (Paice, Glover, Gillan) plus Don Airey who was in Rainbow and replaced Jon Lord on keyboards and Steve Morse on guitar who replaced Ritchie Blackmore in 1994.  So the 'technical build' of this group is excellent.

See: The Web Developers SEO Cheat Sheet

4. Give your audience what they want

Looking around, I suspect that if a bomb had gone off in the Hammersmith Apollo you'd wipe out good percentage of senior accountants, IT directors and civil engineers in the South East.  This isn't an audience looking for the latest new sound of swingin' London.  We don't mind a bit of new stuff, but what we've been searching for and want are the classics.  So they serve them up to us.  Although Don Airey didn't do the Terrahawks theme tune...

See: 10 Steps to Advanced Keyword Research

And if you don't agree with any of this, then the final thing that Deep Purple have taught me about SEO is that if you write a blog post with the title "What Deep Purple can teach you about SEO" then people will read it. 

On a more serious note, applying SEO techniques and principles to real world examples does allow you a different perspective on the tools, tricks and tips we use and immerse ourselves in every day (Learn Link Building from a Will Ferrell Movie?).  It allows you to think of good analogies when trying to explain often abstract and difficult concepts (Explaining Google's Algorithm...) to clients who don't really understand what it is you're going to be doing for their money.  Or trying to explain to your mum what you do for a living.

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*I may be stretching the definition of the word 'much'