I wouldn't assume that "passing" traffic is always up nice and high. They may be operating at a lower level due cloud, operational reasons or just because they want to.
The AIP states that when an aircraft is leaving controlled airspace, they are to make a broadcast on the area frequency with their position, level, intentions. eg; "Traffic to the North of XXXX and Melbourne Centre, ABC is 20nm to the north of XXX passing F180 on decent, joining a 10nm final rwy XX, estimating the circuit at 15."
So if you are on your way into the same field and monitoring Area, you will know about that traffic. If you're only monitoring the CTAF, you won't know about it till it's potentially a conflict. That isn't to say that there may already be a conflict, but it's just one more safeguard.
That's just one example, there are many other reasons why monitoring the Area frequency is important.