I don't think kits are necessary, but they can help you get all the supplies you need without leaving things out. I still sometimes forget to order some small piece of hardware, like a buckle, and kick myself when I have to pay shipping for just that (or worse, convince myself to buy other fabric too to make it "better").
The best part of kits and guides is that they give you nice step by step instructions so you don't forget one small step, like adding a loop for shockcord as you sew up the side of a backpack.
The only problem with kits is that I feel they force you to purchase more expensive fabric than you may have to. Thruhiker has great kits, but they have expensive fabric, for example. I prefer to purchase supplies from DIYGearSupply instead when I am prototyping or trying something totally new, and purchase their seconds fabric. Also, Dimension Polyant almost always has seconds for sale at $3 or $4 dollars a yard. Even buying 10 yards (their minimum to avoid a $20 cutting fee) is barely more than a yard of Dyneema X, and these can be good fabrics for packs.
As for supplies, the things I often use are:
Markers of some kind. I often use sharpies, but that is very permanent, and I am open to other options.
A couple of yard sticks.
A right angle ruler. I love this thing. It helps me create straight lines so much easier than anything else I have used before. Mine is just a steel one that is 1'x 10", I think.
And if you haven't sewn before, one of those beginner kits with fabric scissors, pins, a 60" tape measure and seam ripper are nice. If you are anything like me, get a seam ripper. I use it a lot. Especially when I am trying new projects.