Why does one type of motor have a Plenum and another doesn't? What does it do for my motor? Is it beneficial?

Just wondering

 

"A chamber that holds air....at positive pressure.."

Just part of the design of the intake. Which engines are you comparing?

 

Positive pressure? Bit it supplies vacuum pressure to the FPR and brake booster

My friend has a 351w with a Ken belle blower and he doesn't have a Plenum. I'm sure Ford could have originally designed my 351w to not have a Plenum but for some reason they added it.

 

Two different engine set ups.
The kennebell does not have much of a plenum, but it does have one. It forces the air into the engine so you really don't need much.
The plenum is helpful for tuning the powerband. Big short runners are good for high rpm and power, where long narrow runners promote velocity and low end torque. The plenum helps with the transition from throttle body to runner. Bigger it is the less the rear runners starve or fight for the air. It's just a transition from tb to runner really. Sometimes the factory puts a huge one on and sometimes a small one.
The 300-6,302,351 have big huge plenums and long runners, and yet the 2.3 has nearly zero plenum (more of a transition from one big to 4 small) and huge runners on the dual plug ranger, and fairly big for its displacement runners on the older 2.3. Ranger 2.9 and explorer 4liter have a decent size plenum. It's all application and how the original engine designer decided to get the desired hp output. Doesn't always make sense though.
Trucks like long runners to promote velocity and cars usually like big short runners for upper end power.

 

I like the look of the Plenum I have, after seeing my friend's setup, I think it looks better. This still doesn't give him an excuse as to why my truck is faster than his.