。
Promises are made to be broken.
Literal
Promise [topic-は] be-broken-[passive-られる] [purpose-ために] exist.
~ために exists means 'exists for the sake of' — here combined with the passive 破られる ('be broken') to form the ironic aphorism. ~ためにある is a compact way to attribute a purpose/destiny to something. The sentiment is famously not Japanese in origin but a common translation of the English saying.