The most important things are done through tubes. Proof: genitals, pens, and guns.

☛ Sudelbücher by Georg Christoph Lichtenberg, tr. Harry Zohn, [1775-1776]1984, Sudelbuch E, §35. Keep reading for a more detailed account of quote’s origin.

Lichtenberg’s remark also applies to the Internet since, as everybody knows, the Internet is a series of tubes.
The quote reminded me of a passage from the Tao Te Ching (also known in English as the Dao De Jign or Daodejing, and in French as the Tao Te King):

1. Thirty spokes unite in one nave and on that which is non-existent [on the hole in the nave] depends the wheel’s utility. Clay is moulded into a vessel and on that which is non-existent [on its hollowness] depends the vessel’s utility. By cutting out doors and windows we build a house and on that which is non-existent [on the empty space within] depends the house’s utility.
2. Therefore, existence renders actual but non-existence renders useful.

There are numerous translations of the Tao Te Ching. The one used above is one of the earliest one (1880) by D.T. Suzuki and Paul Carus. One can compare between 11 English translations at Duhtao.com.

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The source of the Lichtenberg’s short aphorism calls for some precisions. It comes from his Sudelbücher or “Waste Books” as they are known in English. They were composed of 11 notebooks, each identified with a letter from A to L (I was omitted). Those notebooks were filled with short and long thoughts, ideas, book references, quotations, etc. Today, he is considered as one of the greatest German aphorist:

German scientist and man of letters Georg Christoph Lichtenberg was an eighteenth-century polymath: an experimental physicist, an astronomer, a mathematician, a practicing critic of art and literature. He is most celebrated, however, for the notes he collected in what he called his Waste Books. […]
Lichtenberg’s Waste Books have been greatly admired by writers as different as Tolstoy, Einstein, and André Breton, while Nietzsche and Wittgenstein acknowledge them as a significant inspiration for their own radical work in philosophy. (from the back cover of the English translation by R.J. Hollingdale: see below for more information)

The remark about the importance of tubes comes from the fifth notebook or “Sudelbuch E”. Two of the sources I found identified the maxim as “E-35”: see Zeno.org and staff.hs (each of those two websites also offer a free access to a sample of Lichtenberg’s Sudelbücher in the original German text: here and here).
Originally the Sudelbücher were composed of approximately 8000 entries spanning over 1500 pages (Faz.net: “Mein Lieblingsbuch: Sudelbücher“). However notebooks G and H were lost or destroyed, as was most of notebook K.
A small selection of those entries (about a thousand) were translated to English by R. J. Hollingdale as The Waste Books (New York Review Books, 1990; Amazon, Google books, read a review of the book). That’s about ⅛ of what is actually available to German readers.
In this perspective, it’s worth mentioning that the quote on the importance of tubes is not part of R. J. Hollingdale selection: one cannot find it in “Notebook E” of the Waste Books. Instead, I believe it was brought to the attention of English readers as the epigraph for an essay by German journalist and satirist Kurt Tucholsky: “The Social Psychology of Holes” (originaly published in 1960). The original German text can be find online: “Zur soziologischen Psychologie der Löcher”. An English translation by Harry Zohn was first published in 1984 in the book German Satirical Writings: Wilhelm Busch and others edited by Dieter P. Lotze (Continuum Books, reprinted in 2001). The same translation also appears in the book Germany? Germany! The Kurt Tucholsky Reader (Manchester: Carcanet Press, pp. 100-101; Amazon).
For those interested, the Stanford Encyclopedia of Philosophy has quite an interesting entry on the subject of holes:

Holes are an interesting case-study for ontologists and epistemologists. Naive, untutored descriptions of the world treat holes as objects of reference, on a par with ordinary material objects. (‘There are as many holes in the cheese as there are cookies in the tin.’) And we often appeal to holes to account for causal interactions, or to explain the occurrence of certain events. (‘The water ran out because of the hole in the bucket.’) Hence there is prima facie evidence for the existence of such entities. Yet it might be argued that reference to holes is just a façon de parler, that holes are mere entia representationis, as-if entities, fictions.

I first became aware of Lichtenberg’s aphorism on tubes via Will Schofield’s blog 50 Watts. It’s worth a visit: there’s an excerpt in English from Tucholsky’s essay along with a pastel drawing by Czech surrealist artist Eva Švankmajerová.