Volatility in SERPS means people are often looking for alternative ways to generate traffic without actually paying for it i.e. advertising. So, what are all other ways to generate free traffic, constant traffic, which is significant enough to become a viable alternative to search engine traffic.
Ranking well in all the search engines is one way to diverify some search engine risk i.e. if you stop ranking well in Google then you still have significant traffic from the other two, but what are all the non search engine traffic.