What is the purpose of dead end neurons in this diagram? I was reading over some sources about ANNs, and here I found this diagram of a Recurrent Neural Network (RNN, the source calls it a feedback neural network).


I noticed that a few neurons in this diagram lead to other neurons that are dead ends (have no connections to other neurons). For example, neuron U5 leads to no other neurons, and U9 has a connection to U4, but U4 doesn't have a connection to any other neurons.

Is there a purpose for U5 and U4 to even exist?

Or am I missing something bigger here?
As said in the comments, $U_4$ and $U_5$ are not used to compute the values of the output neurons. 

However, in some cases one may allow the connections between neurons to change over time, in which case $U_4$ and $U_5$ may be later used to compute the values of the output neurons. Seanny123 gives biology as an example. For another example, you may want to look at the  literature on neuro-evolution, i.e. optimization of neural architectures with evolutionary algorithms, which may allow changing the connections between neurons over generations (e.g. that's what I did for my master thesis:  Franck Dernoncourt. "The medial Reticular Formation: a neural substrate for action selection? An evaluation via evolutionary computation.". Master's Thesis. École Normale
Supérieure Ulm. 2011., though I may have coded the mutation and crossover operators in such a way that dead-end neurons don't appear, I don't recall).