In the early days (or years) of a SaaS product you are fighting for product market fit. Even when you know without a doubt that your product serves a purpose that people are willing to pay for, there's a long road between Minimum Viable Product (MVP) and a mature piece of software.
If it all it takes to onboard a new customer is filling in a couple of fields on a form and choosing some configuration options - whether done by the customer or one of your customer success team - that's Product Delivery.
But if you're doing bespoke development, adding features just for them, building new reports, or even doing on-site training - that's Project Delivery.
They may seem similar at first. After all, either way, you have to deliver something to the customer. This may be compounded if some of your customers want a product and some want a project.
But while treating product delivery like a project may result in some additional paperwork, but otherwise no harm done, trying to perform project delivery with a product delivery toolset will result in a very disappointed customer.
Projects need plans. They need tooling that can show you when things are going wrong. They need discovery, goals, risks and a clear list of responsibilities.
Products just need billed. Which is why everyone just wants to deliver products.