There have been a couple of posts and questions on MathOverflow about the proofs of the following two facts:

**Fact 1**: if $X$ is a topological space, then $\pi_k(X,x)$ is abelian for $k\ge 2$.

**Fact 2**: if $G$ is a topological group, then $\pi_1(G,e)$ is abelian.

Both facts can be proven using the Eckmann-Hilton argument, which is a cool algebraisation of two more topological proofs that one can get by staring for sufficiently long to a couple of pictures* (after drawing them): while, up until two hours ago, I thought that the two proofs were actually distinct, Ryan Budney proved me wrong and showed me the connection (see comments below)

Now, to the question:

Is there any other example of an incursion of the Eckmann-Hilton argument into the realm of topology? Is there any other such application outside category theory/algebra?

I would like to see some results for which no proof is known that doesn't make use of the E-H argument, or such that any proof avoiding the argument (even somehow disguised) is significantly longer/harder.

**EDIT**: Thanks to Ryan Budney for pointing out how the two kind of proofs are actually the same proof, and to Tom Goodwillie for making me realise that the question was a bit too rough in an earlier version.

* One such picture is linked above. The other one is just a square with the diagonal, representing the map $(s,t)\mapsto (f(s),g(t))$: this picture gives homotopies between $f\cdot g$ and $f*g$, so they recover as much information as the E-H argument produces, see the question linked above.

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