My grammar book says "The use of 'ser' with an adjective denotes that the speaker considers the quality signified by the adjective an essential . . . . component of the noun." I think the idea is that your are emphasizing the current 'newness' of the shoes, rather than "Yeah, they are new now, but they'll wear out."
Ser is used for qualities. It least thats what i have be told by a native speaker...