In a recent interview with Wired, when asked about NSA activities and data privacy in general:
Gates: " ... There are legitimate reasons for the government to watch what’s going on, particularly with nuclear and biological weapons. So it’d be nice if there was a way that some part of the government that we really trusted was looking at that information. Right now, people are going, “Oh my gosh!” and you wonder—did they not think anything was going on? But it’s probably good there is now an explicit conversation." [1]
He also has, or at the very least had, a Top Secret security clearance. [2]
While the purpose is debatable, one of the most likely reasons was so he could receive State Department briefings prior to meeting with foreign leaders and businessmen. A lot of that information would likely have come from NSA intercepts.
Another purpose could have been to facilitate Microsoft's cooperation with the NSA on software backdoors, cryptographic or otherwise.
[1] http://www.wired.com/business/2013/11/bill-gates-bill-clinto...
[2] http://www.outsidethebeltway.com/bill_gates_has_top_secret_c...