I tend to believe wood knocks are used in a range of manners to convey different messages based upon context.
Single loud wood knocks, coupled with a loud whoop or other vocal, could be a "tag-up" if you will. Maybe they ask the question "is there any body out there?!" The wood knock portion of a knock/vocal combination could serve two purposes, the first being punctuation "!", and the second being confirmation (that I am indeed a member of your species, and not some other animal vocalizing in the wilderness).
Softer wood knocks seem to be used as a means of keeping track of each other when in close proximity, and/or when danger may be near. Making a milder warning knock that your buddies a few hundred feet away can hear seems wiser than a loud bash, that would also alert the source of danger to your presence.
Here's a recording that includes a soft whoop, and subtle wood knocks, that might be an example of this kind of situation:
Then even softer knocks may be used for still stealthier communication, possibly when danger is very near, or tension is high. I recorded some fast, soft "stick" knocks just seconds after something moved in the forest near me, and grunted audibly. The knocks sound very tense and can be heard at the very end of this recording:
And then there seems to be the intimidation type of wood knock, which forum member uluax actually observed being made in his remarkable class A sighting from Colorado. I don't have the link to that report handy though.
So in general, I don't think we should attach just one reason or meaning to the existence of wood knocks. Their use is probably more complex than we currently understand, and only time spent recording, studying, and learning them will improve that understanding.
But in terms of a component of communication, the inclusion of wood knocks to enhance vocals is a fascinating advance to consider. A rudimentary technology to be sure, but the mastery of a technology none the less.