。
Refrigerators are useful for preserving food.
Literal
Refrigerator [topic-は] food [object-を] preserve [for-のに] is-useful.
~のに ('for the purpose of X-ing') is the purposive/functional のに — distinct from the contrastive 'although' のに. 役立つ ('to be useful, be of help') takes a nominalized purpose clause with のに. Standard structure for 'X is useful for Y-ing.'