blog.dir directory in wp-content folder required ?

i was reading in the book "PROFESSIONAL WORDPRESS:
DESIGN AND DEVELOPMENT" that "The first step is to create a blogs.dir directory in your /wp-content directory." but i saw no mention of this here >

here is the exact book on amazon for your perusal :

Is this necessary ?
If so why /or/ why not ?

Any kind advice on this would be greatly appreciated.

Kind regards,