The two-space norm dates back to the days of typewriters. When using a typewriter, it makes sense to leave an extra space to indicate that the sentence has concluded. Today's word-processing software, on the other hand, makes typefaces proportional, which is why we only need one space. However, many people (including this author) still prefer to insert two spaces instead.
This is not to be confused with paragraph indentation. Paragraph indents are used to indicate where one paragraph ends and the next begins. There should be no space between consecutive paragraphs unless specifically indicated. Two spaces in a row will make most text editors emit an error message because they do not know how to interpret such a sequence.
There are two reasons why someone might insert two spaces instead of one: either as a way of indicating that a new thought has begun or as a way of avoiding printing a single space when writing by hand. Although computers can detect single spaces just fine, many people find them hard to read so they insert two spaces instead. This is especially common in formal writing where every other sentence or so becomes marked with two spaces.
In conclusion, the practice of inserting two spaces instead of one exists and has existed for quite some time. It is still present today in various forms.
Before word processors, two spaces after a period were necessary and taught as right if you learnt to type on a typewriter. Because the spacing between words on a typewriter was irregular, the extra space was required to define the beginning of a new phrase. This is called "paragraph formatting" in modern terms.
In today's world where every document is typed up by someone who didn't grow up with a keyboard, this regular paragraphing is lost. So, many people ignore it completely, while others add a bit of space between sentences but not after each and every sentence as there are no periods to mark off paragraphs.
The reason for requiring a second space after a period is that when you print out a document from within a word processor, it will usually do everything it can to avoid having to set extra paper space. If it doesn't detect a second space after a period, it will assume that you want to have something written across both pages. Of course, this isn't any problem for a document that's only being printed out for yourself, but if you were sending it to another person then they would need to provide an option for having their name appear on only one page!
So, single space after a period is for typewriters, double space is for printers.
Monospaced type produces text that seems "loose" and uneven; there is a lot of white space between letters and phrases, making it more difficult to notice the gaps between sentences instantly. As a result, the two-space rule was adopted on a typewriter; an extra space following a sentence makes the text easier to read. This is known as "the pause rule."
In modern times, monospaced type is used in computer programs to show labels or commands within a document. These elements are referred to as "monospaced fonts." The term "monospacing" means printing or typing single spaced lines of text.
The reason we see so much white space between words is simple: it makes reading the text easier. If all the letters were jammed together without any room between them, it would be hard to tell one word from another or to recognize individual letters until they form a sequence, such as a word or name.
He argues that the practice of two spaces between sentences dates back to the days when everyone typed on manual typewriters. Because of how typewriters handled proportional spacing, typewritten manuscripts with two periods between sentences were simpler to read. This idea is supported by the fact that many early typewriter keyboards lacked any form of space bar; instead, users indicated where they wanted a break by pressing two adjacent keys.
In today's world of computer-generated text, two spaces after a period are not necessary for readability or clarity. However, many law firms continue to use this formatting style as a way of indicating what he calls "the end of an argument" or "the end of a paragraph." He says that using these formatting styles prevents readers from having to scroll down to find out who is speaking or what was said next.
The use of two spaces after a period has been adopted by some legal publications. For example, The Bluebook: A Uniform System of Citation recommends printing names in sentence case with no punctuation except for periods at the end of sentences. It also suggests putting two spaces after each reference number.
So, yes, lawyers use two spaces after a period as a way of signaling the end of an argument or paragraph.