Over a billion people or 15% of the world's population experience some form of disability. We leave a lot of people out by not providing accessible services.
If you create accessible content, you can also reach a lot more people. The content is more readable if you reduce the number of unusual terms, acronyms and abbreviations. This will also help people in other, non-native, countries to understand your content. Companies can improve their brands by showing they care about accessibility.
Yes, some companies are required to be accessible. Check the accessibility requirements in Finland page.
The requirements set a target everyone should be striving for. But those should only be considered as the minimum requirements and not the end goal.
It is easier to take accessibility into account from the beginning of the project. If you implement accessibility later, it will likely be harder and take more time. And thus cost more money.
But don't worry, you can also start improving the accessibility of your existing content in smaller, more manageable pieces. Even small improvements can help a lot. You might not always be able to get 100% accessible content, but you should strive for as accessible content as possible.