In computer science, it is sometimes deemed necessary to distinguish between the data and its representation, to be able to formulate thoughts like "lines of text are stored in computer memory as [explicit] strings of characters" (as opposed, for example, their offsets in a file, etc).
The word "string" comes naturally for that synonymic, but not quite equal, usage, as, for the EngishEnglish noun "string", Sense of "a number of objects arranged in a line" first recorded late 15c. (Emphasis mine - L.)
Explicit occurrences of the phrase "string of characters" in the 19th century books can be found in abundance; for example,
This literary trifling is obviously quite useless as a means of indexing for reference , unless the whole string of characters be learnt by rote
is quoted from Notes and Queries on China and Japan - Volumes 1-2 - Page 74, 1867.