Setting up product catalog has always been time consuming tasks for business owners. To add product to appear selection for salespeople in line item entries, it WAS mandatory to add it as Price List Item.
With 2019 release wave 2, it is optional. You can add any active product in line items grid on Opportunity, Quote, Order or Invoice even though it is not available in the price list. This will be helpful for users to put ad-hoc prices for products.