While making websites accessible can lead to more time spent in development, there are many reasons why you should be making your application accessible.
Multiple studies suggest that around 15-20% of the population are living with a disability of some kind1. In comparison, that number is higher than any single browser demographic currently, other than Chrome2. Not considering those users when developing an application means excluding a large number of people from being able use it comfortable or at all.
Sites that employ techniques to make their site accessible will guarantee that it matches a minimum standard of usability for everyone. Features originally intended for accessibility are often appreciated by all users, voice control modes for phones being one such example.
In addition to this, accessibility techniques such as semantic markup help search engines understand the application better, leading to improved visibility.
Countries around the world have different accessibility rules centered around compliance of two main sets of guidelines: Web Content Accessibilities (WCAG) 2.0 and Section 508. Not being in compliance of these rules exposes you to liability.
WHO. "Report on Disability". 2011
Eurostat. "Prevalence of basic activity difficulties or disability". 2012
US Census Bureau. "Anniversary of Americans with Disabilities Act: July 26". 2012
Statcounter. September 2016