I've always wondered why Gmail doesn't sort search results by relevance, just like Google Search. Gmail sorts results by date, so you can quickly find recent messages that match your query, and that's pretty useful, but sometimes you want to find an old email from a friend or a long-forgotten conversation about modern architecture.
Google Inbox has just added a "top results" section that shows the most relevant 5 results for your query. It's not perfect and it's easy to see why this doesn't replace the standard "sort by date": ranking email conversations is harder than ranking web pages. There are no links between messages, many messages are pretty short and are only important for a short while.
In addition to top results, Inbox now also shows quick answers extracted from your messages. "Now when you're searching for a frequent flyer number or shipping status, Inbox will show it at the top of search results – no more digging through individual emails to find what you're looking for."