There are probably cases where plain text email is enough (linux kernel development seems to be doing fine), but once you enter corporate world HTML and all its goodies bring a lot of benefits.

(HTML) "Please have a look at this table (table pasted inline to email) and compare the results with previous run (another table inline)"

(plain text) "Please have a look at attached table (table1.xlsx) and compare the results with previous run (table2.xlsx)"

Sure, both bring the same information but the former gives you all the information immediately.