They're still going to have the descriptions with the stat blocks I assume, and those explain exactly what the monsters are without diving too deep into the lore presented in the rest of the books.
The reason why they are publishing in that manner is that they are wanting to update the stat blocks to the new style. And also, it's basically going to be a Monster Manual II for this edition, so a new DM that just wants more monsters than just what's in the MM will get more bang for their buck, instead of having to buy two books with lots of extraneous lore they may not want or need.