The anthropic principle (AP) is an explanation of our being observers to the world. The universe could have been empty if things were to have evolved in the universe differently and this is one of the central topics that the AP tries to explain. It does so in both the weak anthropic principle (WAP) and strong anthropic principle (SAP).
The WAP wasn't very satisfactory for a number of reasons (mainly physical constant coincidences), and so the SAP was also enunciated by Carter as an explanation tool. The use of observers in the SAP was another means to explain the cosmic coincidences in a manner which didn't insult our intelligence (as some felt that WAP asked us to do). Since the SAP was created to answer the question on why it is that we are here versus the obvious possibility of us not being here to make this observation, the focus was again on observers.
When Barrow and Tipler (Barrow, J. D. & Tipler, F. J. 1986. The Anthropic Cosmological Principle. Oxford: Clarendon Press) elaborated on their interpretation of the AP, they gave the SAP a further breakdown:
(1) There exists one possible Universe ‘designed’ with the goal of generating and sustaining ‘observers’.
(2) Observers are necessary to bring the Universe into being.
(3) An ensemble of other different universes is necessary for the existence of our Universe.
The term 'observers' for (2) was additionally useful since this went along with Eugene Wigner's suggestion that quantum rules require an observer to collapse the wavefunction (and ultimately God to collapse the wavefunction of the universe). If observers perform that role as collapsing wave functions, then this is another possibility that fits in well with SAP. Hence, the observer emphasis had an unforeseen benefit.
Of course, the universe didn't need to have observers, but what we want to explain is why it happens to have observers.
Warm regards, Harv