The Old Testament is quoted many, many times in the New Testament.

Number of quotations from the OT: 1644
NT uses OT on average once every 3 verses!
Most frequently quoted books: Isaiah (412) , Psalms (410), Exodus (240), Genesis (238)

What are the implications of this?

It helps to justify that Christianity does naturally follow on from the Hebrew Bible.
Christianity must be one of the very few religions, coming after a previous religion, that fully accepts what came before it. So many religions come after another religion, say they are the true fulfilment of that religion but that the writings of the previous religion have been changed.

Religions which make these claims

Islam –
Manichaeism – Mani claimed to be the Paraclete of Truth  (claimed bible changed couldn;t produce it)
Jehovah Witnesses – Claim the Bible is correct but only use their own ‘translation’’s_Witnesses