One can appreciate the importance of glue by an analogy with carpentry. A chair can be made quite easily by making the parts - seat, legs, back etc. - and sticking them together in the right way. But this depends on an ability to make joints and wood glue. Lacking that ability, the only way to make a chair is to carve it in one piece out of a solid block of wood, a much harder task. This example demonstrates both the enormous power of modularisation and the importance of having the right glue.
Now let us return to functional programming. We shall argue in the remainder of this paper that functional languages provide two new, very important kinds of glue.
(After that, check out this much more populist and recent approach to the same topic.)