What makes a clause essential or nonessential?

Question

So I'm currently practicing how to use commas properly and have just come across the following example:

Jillian, who had worked in the dress shop all summer, hoped to work there again during the Christmas holidays.

As far as I know, commas here are used to set off a non-essential clause that doesn't add any essential meaning to the sentence. One would argue that commas are not needed here since the clause between the two commas is essential and removing it will affect the meaning of the sentence. Am I right?

Reference: owl.purdue.edu

Answer 1

Let us consider the relevant sentences:

• (1) Jillian, who had worked in the dress shop all summer, hoped to work there again during the Christmas holidays.
• (2) Jillian hoped to work there again during the Christmas holidays.
• (3) Jillian hoped to work in the dress shop again during the Christmas holidays.

Sentence (1) adds additional information to (2), but the basic meaning is the same. Sentence (3) adds back the more important part of the meaning, using a different structure. Sentence (2) works well only if some previous context tells the reader what "there" refers to, but it is still grammatical without that. Sentence (3) avoids that problem. It is thus clear that "who had worked in the dress shop all summer" is a non-essential clause, and should be set off by commas.

Answer 2

Removing the commas just sounds unnatural, but is very common in casual speech. However, you'd see the commas very often in writing.

"Jillian who had worked in the dress shop all summer hoped to work there again during the Christmas holidays"

This sentence just sounds very rushed, it's not wrong nor does it change the meaning much.

You could rephrase to:

"After working the whole summer at the dress shop, Jillian hoped to work there again during the Christmas holidays."