**For example, what is "Product Qty Maximum"? **
That's the maximum number of this product that can be added to the cart per order.
**Why is it important? **
It's akin to "Limit 3 per family" which you might find at your local supermarket. The decision is entirely up to the storeowner.
**What is "Products Model", **
That's the model number you've assigned to the product. It will be displayed on the product page and at checkout.
**and is it important to specify this? **
That's up to you.
**What is "Products Shipping Weight" **
That's the weight of the product, used by your shipping modules to calculate shipping costs
**and how will this affect checkout? **
If not specified, shipping costs cannot be calculated based on weight. You'd have to use per-item or flat-rate, etc instead.
Do I need to specify it?
No, but the consequences will be as described above.
**How would I add a product with limitless quantities? **
Put a very large number, or put zero and turn off stock-tracking so people can checkout regardless of qty on hand.
(These are just examples... I don't need answers to these questions.)
As you can see, they're pretty much self-explanatory, and pretty much mean what they say.