Points that support creating a separate database for source(which is usually coming in through an API Request)
The value of having a separate source database is that if your transformation process fails, you don't have to go to the external sources to pull the data again.  You can simply fix the transformation issue and start at the transformation step, skipping the data load.

If the time that is required to pull the data from the external sources is insignificant, then it may not be worth the extra effort.  You simply have to weigh the various factors to determine what's best for the project:


How long does it take to extract the data?
If the transformation fails, how long will the original data still be available if you need to extract it again?
How quickly will you be able to address any transformation issues?
Is this a short term process, or will it be running indefinitely?  If it will be running indefinitely, you'll want to spend more time engineering so that you spend less time fixing problems, especially if someone else will be managing it eventually.


Using a source database could also speed up your testing processes since you can test the load of the source database and the transformation processes separately.