Weed N Feed is the proverbial product marketing dream in my opinion.
Think of it in terms of a multivitamin for yourself. A product containing various beneficial ingredients which may or may not be needed by your body in the provided dosages or if at all.
Back to lawns. Does your lawn currently HAVE weeds? If not, you don't need a herbicide (the WEED portion of the product).
Does your lawn require nutrients? (the FEED part). Try not to get caught up in the race to have the "greenest" lawn on your street. Higher Nitrogen formulations for a greener lawn mainly encourages short term foliage growth which equates to more frequent mowing(s).
IF you have weeds you don't want, then use a herbicide, if allowed in your area. (Pesticides are largely banned for homeowner use in most municipalities of BC where I live)
IF you feel your lawn needs feeding then buy a quality product recommended by a LOCAL garden center or landscape pro designed for your particular plant zone.
Save your money and stay away from the "one size fits all" products.