There are at least two reasons to make podcasts accessible. First, they reach more people, and second search engines like it. It’s the same reason why descriptions of images are important. Both reasons help grow your audience. A third reason is that transcripts help you to find the content at a later date. If you have transcripts for every show, you can search and reference what was discussed on your show at any point. In essence:
It’s the right thing to do
People with disabilities benefit
Other people benefit
You benefit – Your content is indexed
Your reach increases
There may be legal requirements
The Podcast Accessibility website has more detail on the list above about making podcasts accessible and why it is important for everyone. It also has other useful information apart from transcripts. It’s an easy read.