Why optimize for voice search? We all speak things differently than we write them, and search engines look at voice search differently than a text search. Long tailed keywords are more important than ever with voice search and search results for voice tends to single out one answer. That makes is vital you show up as number one. Your location is also a part of voice search. If you ask for a restaurant the search results will first look locally.
Google estimated last year that 1/5th of search is now initiated by voice. Voice search tends to be different from written search and Google looks at both differently. A greater proportion of young adults use voice however all age groups are showing more comfort with using voice search and the numbers are growing. This trend it expected to continue as voice search becomes faster and more accurate than text search.
So optimization for voice search has become a new and important part of SEO work. And if you are trying to out-do a competitor this could be a niche you should take advantage of before they do.