I apologize if this is to basic for MO, but it just seems a bit to advanced for SE.


Let $\text{Top}$ be the category of topological spaces and $\text{sSet}$ the category of simplicial sets. There is an adjunction
$$
|\cdot|:\text{sSet} \to \text{Top}:Sing
$$
which is a Quillen equivalence with respect to the standard model category structures. One of the fundamental results of the general theory is that this adjunction induces an equivalence of the homotopy categories (the localisations with respect to weak equivalences). It is often described in textbooks as an application of model category theory.


It seems to me that in order to prove the equivalence of homotopy categories, by abstract nonsense, one needs only to prove that the counit of the adjunction, (i.e. $|Sing(X)|\to X$) is a weak equivalence. My first question is



> 
> Is there a direct/slick proof that $|Sing(X)|\to X$ is a weak equivalence without using model category theory? 
> 
> 
> 


**Remark**: It is easy to see that it induces isomorphism on homology, since the simplicial chain complex of the first is exactly the singular complex of the second, but this is of course not enough.


Frankly, I would be very surprised if the easiest proof of this fact indeed requires model category whose machinery is rather heavy to set up. If my intuition is correct, it seems that mentioning it as an application of model category theory is a bit misleading. I understand completely (at least on the philosophical level) that model structure gives you much more than just equivalence of the homotopy categories, but I would be glad to have a pedagogical answer (to myself) to the following question:



> 
> What nice and easy to describe applications model category theory has?
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Equivalence of homotopy categories and model structure theory
You can find a "direct proof" in the paper by Curtis Simplicial Homotopy Theory (Advances in Mathematics, Volume 6, Issue 2, April 1971, Pages 107–209). The main ingredient is the use of barycentric subdivision.


