The question isn't "is it needed." The correct question is will marginal profits from advertising exceed the cost of adverting. Done well, undoubtedly the answer is yes.
Just like Model S/X would sell itself. As sales went from 100k/year to 57k/year.
What really caused the S/X sales to drop was Tesla’s removal of the lower variant. Many of those sales most likely went to the higher end model 3. Newer tech.
Now the S/X are back to being flagship vehicles.
And GM’s ads will help.