The point is that both can be required. You can have the most sophisticated user emulating browser, bit if all you have access to run it on are low quality IPs that have been blocked or that are often used for abuse, you won't get far. You can have residential IPs and if you're just wrapping curl, you also might find you're blocked.

Together, there's little to detect different than a regular user. The reason why the residential IPs is given heavy importance though is that it's the one part that costs a lot of money if you need enough of them you need to use a proxy service and you transfer a lot of data. Entry level pricing is over $15/GB for high quality services.