They include it to point out that it's OK to do it. Those who are new to the game might not feel comfortable monkeying with the rules like this if it were not explicitly shown as an option.Which of course begs the question of why even include it in the book in the first place.![]()
Newer DMs also might not have a good sense of what "wing it" actually entails. The examples of how to balance eladrin and aasimar show some important guidelines and things to think about when building a new race -- without this advice, the novice DM or player is bound to go way overboard in trying to make the new race cool and exciting rather than balanced.
They couldn't possibly spell out every possibility for racial abilities. That is an infinite pool to draw from, and the book only has a finite number of pages. As with every other aspect of 5e, they've given us a good general idea of how to do it and included some useful examples. If, after reading these examples, you still feel completely out to sea in terms of balancing a new race, then you probably shouldn't be trying to create a new race. If I were in a snarky mood, I'd say you shouldn't be trying to DM at all.