Well, to give you a sense of their importance within music, take John Covach's textbook What's That Sound: An Introduction To Rock And Its History (I'm looking at the 2006 edition at the moment). This textbook devotes about 550 pages to a history that focuses on the rhythm-heavy capitalist music of America and the UK from 1955 onwards. Broadly speaking, within this history, metal gets 10 of the 550 pages, and within that, the British New Wave Of Heavy Metal gets a page. This is, broadly, the subgenre that Iron Maiden get lumped into, and the book broadly sees BNWOHM as being a reaction against the 'mainstream rock that the major labels had been releasing', like punk and new wave, which was briefly popular (with an implied 'in the US') but then overwhelmed by hair metal.

This the entire description of Iron Maiden within that book:

Iron Maiden formed in England in 1976, and like Judas Priest, first hit in their home country with Iron Maiden and then in America with The Number Of The Beast.

So, well, at least they're not literally a footnote in rock music history.

Of course, heavy metal is no longer simply a subgenre of rock, but something entirely different, with a fairly different audience, different musical rationales and aesthetics, and a blizzard of subgenres of its own. The BNWOHM, I think, is more important than Covach paints it, because it's the point where heavy metal becomes thought of as 'heavy metal'. 'Heavy metal' was used as a description of certain musical sounds before the BNWOHM but it wasn't a consistent genre name that everyone used ...until the mid-to-late 1970s, when groups like Judas Priest, from album-to-album, came closer and closer to a distinctive genre sound separate from the (more blues influenced, and more boogie-reliant) hard rock of, say, Grand Funk Railroad (for more on Grand Funk, check your school library...for Billy James' book An American Band: The Story of Grand Funk Railroad).

Anyway, Sound Of The Beast: The Complete Headbanging History of Heavy Metal by Ian Christe sees Iron Maiden as crystallising the idea of the British New Wave Of Heavy Metal - they were sort of the first metal band to arrive fully formed as metal (where Lemmy of Motorhead and Judas Priest took a few albums to really shed their last few slivers of hard rock), who combined the music with a very Hammer Horror film sense of visual imagery, something which also became a distinctive part of metal.

Christe also sees Iron Maiden as being particularly influential in spreading this new idea of heavy metal across to America with the Number Of The Beast album in 1982, which pushed American heavy metal in a harder direction, the NWOBHM in general influencing the American thrash metal scene - Metallica's Garage Days Revisited EP, for instance, featured covers of BNWOHM acts, if not Iron Maiden, and Christe mentions Slayer doing Iron Maiden covers in their early days. And there's a subcategory under Iron Maiden in the index of Christe's book, which goes 'powerful influence of', which has a good dozen entries, mostly referring to 1980s acts imitating the operatic vocal style or the twin lead guitar stylings that Iron Maiden used. Broadly speaking, it's probably fair to say that Iron Maiden (and other acts from the BNWOHM) had a fair amount of influence on metal's path to being its own genre rather than a subgenre of rock (which is why Covach gives them relatively short shrift, because he's writing a history of rock, in which metal only really gets a mention for its influence on the world of rock, rather than a history of metal).