Like ., : works more or less like true. Unlike ., : is a more portable version of true as : is a special POSIX built-in (read: required), whereas true is a regular built-in (read: optional).

Nowadays both : and source (from now on I will use just :) are used in very clever ways:

• Persisting environments (only true for : but not in zsh):

$ unset x; ( x=hi :; echo "$x" )
hi
$ unset x; ( x=hi true; echo "$x" )
 # hi no printed

• Command logging:

% set -x # set -x, makes the shell print out the command before running it
% : Logging message here
% example_command

• Cron job titles

This usage is similar to above, clever!

45 10 * * * : Backup for database ; /opt/backup.sh

• Skipping block of codes:

: << 'SKIP'

your code block here

SKIP

Reference: What is the purpose of the : (colon) GNU Bash builtin?