When selling online you want to make sure that your core processes are as friction-free as possible for customers.
This means when a customer is on a product page, they should be able to easily and clearly recognize the add to cart button.
This button serves as a reminder that, "hey you can buy this thing."
There's one time when you shouldn't have your add to cart button visible on the product page though.
That's when your store's design and copywriting is setup to gently introduce the product.
You see this all the time with luxury goods and high-end services.
You're not told the price until you are sure you want it. Even then, sometimes the seller will hold back the price until they've qualified you.
You've got to really, really want it.
You might not have the luxury of this with your store.
You might want to have everyone buy your product.
But you still can have some of the luxury appearance without all of the baggage.
The way you do this is to add enough images and copy for the product so that when the product page is first loaded, you don't see the add to cart button.
This will give the customer a chance to start reading or skimming and get an idea about what you or the product does.
Then after they've read a bit and scrolled down, they can see the add to cart button.
At this stage they are either prepared to buy, need more information, or don't want to buy.
This won't work for everyone or every product type. It won't work for commodity products.
But if you have something unique and special you're selling. Perhaps something with a story behind it, it can work extremely well.
This reasoning is one part of why Sticky doesn't appear until your customer has already seen and scrolled past the main add to cart button.
It makes it a little harder on my end to code but it makes the customer's experience much more valuable and satisfying. Their desire for your product builds up until they feel like they have to buy it.