org-mode: capture templates - How to use them?

org-mode: capture templates - How to use them?

I've started using emacs's org-mode often, and finally I get to configure
org-capture.
However, I've found in most places that one can configure different
templates by adding the following lines to the .emacs file:
(setq org-capture-templates
'(("t" "Todo" entry (file+headline "~/Documents/Orgfiles/gtd.org"
"Tasks")
"* TODO %?\n %i\n %a")
("j" "Journal" entry (file+datetree "~/Documents/Orgfiles/journal.org")
"* %?\nEntered on %U\n %i\n %a")))
Question
What is the purpose of using these templates?
Could you (please) illustrate their uses with some examples?