This is not much of an answer, but it might help. The category of simplicial sets should be thought of as the free cocompletion of $\Delta$. In other words, it's what you get if you freely take colimits of simplices, and since taking colimits is the most general form of gluing, it's precisely the most general setting for gluing simplices abstractly. 

The essentially geometric nature of this construction is maybe clearer if you first restrict to the subcategory of $\Delta$ on the first few objects. For example, on the first two objects you get the category of graphs. 

Anyway, this immediately implies the existence of geometric realization $\text{sSet} \to \text{Top}$, which is given by interpreting an abstract colimit of simplices as a topological colimit of (geometric realizations of) simplices. If you know that every topological space is weak homotopy equivalent to a CW-complex, maybe the idea of relating the two categories by geometric realization is not so strange.