1) To join two independent clauses: Some travelers like to plan their own trips; others like to use a tour company.
2) To link clauses connected by conjunctive adverbs or transitional phrases: Travelers who use a tour company prefer to have everything planned for them in advance; however, they pay a premium price for that luxury.
3) To separate items in a list that includes commas: He packed a toilet kit that included a toothbrush, comb, and toothpaste; a computer bag that held his laptop, cables, and mouse; and a daypack that contained his water bottle, energy bars, and maps.