When something goes wrong with your phone, you want to be able to call (or more usually chat) with someone that can help you fix it. Google currently offers on-device tech support that lets you call Google Support right from the Settings app if your Pixel gives you trouble, and those calls are going to go unanswered starting in October 2019. That date is, not coincidentally, also when the Pixel is going to stop receiving Android security updates.
The Pixel will also no longer be guaranteed Android version updates after October 2018, two years after it was announced. Putting expiration dates on a product is always a bit of a buzzkill, but I'm happy that Google has given us the dates, and I'm very happy with the dates they've given us.
Most phones are lucky to see Android version updates a year and a half after their release, and the Pixel will see at least two years of them. Most phones don't receive many if any monthly security updates on their phones and the Pixel will see three years of them. Getting support for a two-year-old phone is unheard of, but you'll still be able to call Google and get help with your Pixel for most of 2019, for a phone that came out in 2016. That's pretty incredible.
Google has also updated the end-of-life dates for the last of the Nexus line). The Nexus 5X and Nexus 6P will not see an Android version update beyond Android O this fall, and they will stop receiving security updates September 2018, along with phone support at that same time.