Eventually, player will need items to meet demand because... Waystation (and maybe Megaport). Waystation is required to keep your colony stocked with crew, supplies, and fuel, but only if demand is met. If you want to be self-sufficient, or at least be able to resupply at your colonies, especially once you reach size 8+, then you need items (cores or otherwise) for your fuel production and heavy industry to meet demand.
If other factions sell pristine ships, then buying them makes sense if you cannot reliably produce pristine ships.