In that sentence, you could replace the semicolon with a period and start a new sentence. Or you could use a comma. Or you could use nothing. That raises the question: Why did the writer use the semicolon?
There's only one possible reason: because she could. This quotation did not need a semicolon. It merely presented the opportunity to use one — an opportunity certain writers are all too keen to seize upon. So in this sentence, the semicolon serves only to show off that the writer knows how to use semicolons. That's annoying. It means that the writer was more focused on herself than on the reader.
Semicolons have two functions. They connect closely related independent clauses and they serve as sort of uber commas to manage unwieldy lists. Independent clauses by definition can stand alone as sentences. That means that a semicolon between them is never necessary. Unwieldy lists are, by definition, unwieldy. That means that semicolons aid and abet bad writing.
"Mary is enjoying her new duties, as she handles the staff schedules, a task previously handled by Bob; oversees purchasing, where she has implemented a number of cost-cutting measures, increasing profits by 2%; and plans merchandising for the company's Northeast retail outlets, though Bob still oversees the Southwest."
That Frankensentence would not be possible without semicolons. Instead the writer would have been forced to break the passage into smaller, more comprehensible sentences. In other words, semicolons made this bad writing possible.
The only time semicolons are necessary is in a list of items that contain internal commas: The company has outlets in Bangor, Maine; Indianapolis, Ind.; Key West, Fla.; and Portsmouth, England. Here the semicolons are needed to show that Bangor and Maine form one thing and Indianapolis and Indiana another.