The importance of Tolkien's minor characters

[Guinevere]

Does anyone else have any good examples?

I just remembered two minor characters that nobody else has yet mentioned in this thread.
The two nameless orcs, one a big fighting orc, the other a small "tracker", talking and arguing when Frodo and Sam are walking on the orc-path between the Morgai and the Ephel duath. I always found that conversation fun to read . It even evokes some sympathy with the small "snuffler" orc, who is obviously terrified of the Nazgûl, and would rather go home than work for Sauron.
"You can't even stick to your own folk. Go to your filthy Shriekers and may they freeze the flesh off you!"
They are not important to the plot of the story but, as others said, make it more colourful and realistic. This interlude shows the characteristics of orcs and their behaviour, in contrast to the people on the good side.