Cook has spoken at the annual conference a number of times over the years, although he did not appear in 2014. While Cook has not made any major announcements at the venue in the past, he has at times offered an interesting perspective into Apple's business, so it generally worth paying attention to his comments.
Cook will almost certainly be asked about Apple's blockbuster iPhone sales and earnings in the last quarter, the company's strategy for managing cash and capital return to investors, company philosophy and more, and he is generally fairly open and thoughtful in sharing his ideas on many topics. The company's future product plans are not, however, something he has shown willing to discuss, maintaining a long tradition of secrecy for the company.
Apple CEO Tim Cook Speaking Live at 2015 Goldman Sachs Technology Conference