Note there are hundreds of ways to use the verb "go", and quite a few different ways to use "go off", both in American and British English, of which this is only one!
This is one of the American English colloquial uses of the phrasal verb "go off", which is probably best categorized as, "go off and do-something-that-is-not-typical". In your case, formed in the past tense as "went off and did-something-that-was-not-typical".
As a first approximation, it means they "left their current location" or traveled through some relatively significant combination of space and time, and then did something that was special (or at least not-typical):
Consider the following more simple example:
"Did you hear about Johnny?"
"No! What happened?"
"He went off and got himself married!"
Above is more simply stated, "He got married." The idiomatic phrasal verb conjures an image of Johnny getting up from somewhere, going somewhere, and getting married. That isn't always intended to be absolutely literal, but rather, figurative of the effort required and, equally important, that it was a surprise that he did so. Perhaps nobody (especially the speaker and the person being spoken to) thought Johnny would get married. It certainly isn't typical for Johnny to get married. There is usually a basis for considering that some distance was traveled as well.
Consider the following common use of the phrase, followed by removal of the idiomatic structures:
Why don't you go and get yourself a job?
Why don't you go and get a job?
Why don't you get a job?
In your case sentence, "These Egyptians" refers collectively to the Egyptians over a longer period of time. They developed the science of surveying and used the instruments for practical use. Such a development collectively required some non-trivial movement and effort.
See, it's certainly true that these Egyptians went off and used surveying instruments. (Original)
See, it's certainly true that the Egyptians expended time, energy, and travel to use surveying instruments in ways both practical and novel.
A somewhat better-worded, holistic transformation of the sentence from the vernacular to the formal could be,
Of course, the Egyptians used and developed the science of using surveying instruments.
Even better holistic wordings require greater context:
What is meant by "the essence" of something, such as computer science or geometry? In the case of the Egyptians, while they certainly did forward the advancement of the science of using surveying instruments, more fundamentally, they were formalizing notions about space, time, and mathematical truths.