In some systems source and . are interchangeable command. (POSIX compatibility is swamp)
Anyway source command is good if you need (temporary) setup your environment: e.g. load environment variables, aliases and even request passwords from user with read -s <variable_name> or to load functions from external libraries.
read -s <variable_name>
Usually source is used in scripts(1) but I'm sure that users can find use cases when to use source to setup environment for running shell.
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