One use case at last is geographical performance. One company I was at that switched to Dyn (2008/2009 time frame) had a high performance requirement. They were using F5 Global Traffic Manager prior to Dyn, and it was hosted active-active out of two data centers one on each coast of the U.S. Apparently their customers were complaining that DNS lookups were too slow, part of the (then, not sure if it still is true now) F5 DNS architecture when routing traffic to different geo locations was it required an additional DNS lookup (I forget why), so going to www.mydomain.com resolved to one CNAME which then resolved to a 2nd CNAME then you got the IP of the geo source from there.
Dyn's setup removed one of those CNAME lookups and combined with more geo diverse locations allowed DNS query times to drop by maybe 20-30ms (maybe more I forget now). The lower responses made their customers happy. Though I thought it was stupid just because nobody can tell that difference in performance ("but it shows up in their monitoring" was the response). Whatever.