Have you ever asked yourself the question of why database tables do not have two primary keys? Or maybe you have started a spreadsheet with a choice of columns and rows to only change them later?
One of the little understood properties of programming is that at the lowest level things tend to be single dimensional. So for example, the balanced tree has just one key, and so on. There is theoretical reason for this, but the general fact is that it is much easier to design a structure or algorithm to do one thing than to do two or more.
Object orientation starts with the fundamental idea that objects can do many things. And this naturally leads to the expectation that they can do them equality well. We know from the previous paragraph that this is not true, and yet many do not know this or forget it. You would think that this mismatch between expectation and reality is not really a big problem, yet sadly it is. OO programmers are constantly trying to fit multiple dimensions of abstractions into single objects without understanding that they will never be able do it well without giving more weight to some dimensions over others. Not prioritizing things, not having a clear idea of hierarchy of concepts, leads to a random choice of hierarchy at the deeper implementation level, and leads to redundancies that do not support each other well. The end result is that OO software tend often both to grow and age badly.
Functional programming approaches things differently. A function has only one entry point: from the top. Primitive data structures are all simple (one dimensional) and must be composed to build anything real. The consequence is that with functional programming you need to decide "on construction" how you will interleave the different dimensions of your problem. Unlike OO, you are immediately forced to care about how you will implement things. This is both a good thing and a bad thing. The good thing is that in the end, there is no escape that the deep down implementation of your structures needs a hierarchy, so you might as well take responsibility and choose how you will build your solution. By doing so you can keep things consistent, and consistent does not always mean the same, it may mean that your choice of order of composition is different for different parts of your program but that the unrolling of one part matches the "rolling-up" of another part of the your program. The bad thing about needing to choose early how you split and hierarchize things is that you need to have a lot of experience to do it right (think monads for example). This is why although OO programs tend to not grow well, they are still cheaper to produce as you need senior guys to work functional style.
But now I would like to bring in something different and yet very relevant. Agile development is a lot about prioritizing your requirements and implementing them in iterations. That means that the first level of your design, what might be seen as the first draft of primary keys of your data base, are directly tied to the most important business concept. If you think of it, that is not a bad choice! So to observe that agile development helps structure your design when you do not know better.