В чем разница между joinWith() и with() в Yii?

В чём разница - описано в документации

joinWith() public method

Joins with the specified relations.

This method allows you to reuse existing relation definitions to perform JOIN queries. Based on the definition of the specified relation(s), the method will append one or multiple JOIN statements to the current query.

If the $eagerLoading parameter is true, the method will also perform eager loading for the specified relations, which is equivalent to calling with() using the specified relations.

Note that because a JOIN query will be performed, you are responsible to disambiguate column names.

This method differs from with() in that it will build up and execute a JOIN SQL statement for the primary table. And when $eagerLoading is true, it will call with() in addition with the specified relations.