Summary: Inclusion is critical as a concept because it is at the heart of all UX activities and intentions. Including user behaviors, needs, and desires in design decisions is core to good UX Design. Including users with disabilities when improving or designing for Accessibility is critical. Finally, including target users in regions your product is used is vital for localization UX. Inclusive design is important to understand because it starts with understanding what is important to your audience, not your company or organization alone.
What is the basis of excluding users from a product development process?
There is a stronger tradition in software engineering of excluding users than there is of including users in the design and product development process. This stems from a few root causes:
1. Historically engineers were responsible for user interface design. "We don't use designers let alone talk to users!"