When you have a source information of whatever kind (be it a certificate, a book, a census return, an interview with someone, or whatever) there may be quite a lot of data that you want to enter that comes from that source. Manually citing the same source each time you add an item of data (each name, each event, etc) could be slow and tedious. There are better options. One option is to create a prepared citation in the Citation Window, and then copy that citation and paste it into all the other data items (see How to Copy and Paste Source Citations). Another solution however is to use automatic source citations. See Automatic Source Citation Pane for more details.
See also Source-Driven Data Entry and Prepared Citation, as well as Sources and Source Templates and Source Records,