As a purely practical matter: maybe your ebook reader only supports the format that was discontinued; maybe the ebook has gone out of print or has a change you don't like; or maybe you brought it when there was a big discount and to re-purchase elsewhere will cost you more.

As a broader matter of principle, it proves your property rights over a DRMed ebook are weak compared to a regular physical book; if you had a physical book, the seller couldn't barge into your home and remove it, refund or no.