The main economic difference
With an ordinary available domain, you typically pay a registrar’s registration and renewal charges. With an aftermarket premium domain, you pay an acquisition price to obtain a name someone else already controls, then continue paying normal renewal costs unless the extension has special premium-renewal rules.
Branding trade-off
Premium names can offer brevity, clarity, or exact wording that is no longer available at standard registration. Regular domains preserve capital and may be perfectly adequate when the available name is still strong.
When to upgrade later
Some companies launch on an affordable domain and acquire the ideal name after product-market fit or funding. The risk is that the target domain may rise in price or sell to someone else.