mainly to be able to use labels (see bottom right). However, this feature doesn't work with non-ascii characters, so my nice 中文 label returns an empty list if you click on it.
The Blogger help page says they're working on it.
Another problem is comments. My old comments look garbled because the extended characters are shown as if coded in UTF-8, although they weren't at the time. And, according to the Blogger help page, bloggers who haven't switched to Beta won't be able to leave comments.