Some context: ‘i’ is old web. ’em’ and ‘blockquote’ (along with lots of other tags) are part of a newer version of web mark up (HTML5).
‘i’ (an abbreviation of italics) just says ‘make this display differently’ which is useful for humans reading the text.
Whereas ’em’ and ‘blockquote’ provide the possibility to make the text look different to humans but also attempt to describe the text (to applications that read it). i.e. They attempt to explain why some text is different from other. Some text needs emphasizing. Some words are a quote.
These new tags are an attempt to better organize data and move towards data that can be better processed by machines.