This depends on a case to case basis.
Based on User Research Behavior specific to your form you can select one of the two options
When an Add More button is required.
This button is required when you don't anticipate the user to provide more rows of information under normal circumstances.
In your example you are asking the user to fill up a form for some kind of tax / insurance / CRM / finance etc system. Your assumption is that the user has given their primary income and that there will be few users with alternate sources of income, that too multiple.
Based on User Tests, if there are too few users entering multiple sources of income, use the Add More button.
When an Add More Button is not required.
When you know for a fact that the user is not going to be content on entering just one additional row of data. A classic example of this would be lists. I'll give the example of Trello, where there is no Add More for adding cards.
The same concept is used by Google for Google Tasks, Asana for Projects, Apple for ToDo etc.
Based on User Tests, if almost all users are entering multiple sources of income, use the Add More button.
Although cognitive load is higher when you automatically add rows, it can become extremely irritating to the user to keep pressing Add More in places where it is painfully obvious there is a need to fill the next item.
If you don't know, or you can't perform tests, go with the Add More Button. Its safer.