Need some clarity
 

The manufacturer field. What is the purpose to it?

If I enter something there, it shows up in the description of the product. Lets say for RAM, part number X534534FC , manufacturer Corsair, shows up as

Corsair RAM X534534FC (if I omit the manufacturer it'll just be RAM X534534FC).

The issue I'm having is that if I wanted it as Corsair RAM X534534FC why not just put it all in the part field?

And also would it not be better to have the manufacturer name in the seo friendly url? I would see it more practical there. Easy enough to do it if one puts the manufacturer in the product field directly. Which leaves me a little unclear about this setup. Maybe I'm missing something.

You can do whatever you wish.

The manufacturer field was just how this has been coded from day one. it allows for a separate manufacturer field from the product name or title.