I haven’t read the article so lemme take a jab at it without the need for scientists:
It’s to get all up in there and jizz as close to the cervix as possible?
Lady likes it, makes her want to do it more often, more likely to make a baby?
Am I getting close? Doesn’t sound too difficult to figure out. Now lemme go ahead and actually read it.
Okay?
Mate attraction and rival assessment. In other words, who you appeal to and who you intimidate.
That doesn’t make sense if you compare it to other primates just like they already do in this short article. Other primates accomplish this with e.g. big muscles which show that you can defend and take care of your family.
The question is begged: why the penis? That’s more interesting than the obvious things presented here.