Well the hybrid configuration depends on it and is used essentially for free/busy sharing between your on-prem org and Exchange Online.
With all your mailboxes in ExO, you haven't noticed it however :)
If you re-run the Hybrid Wizard it will look for a valid Federation Cert and throw an error if one doesnt exist.
Of course there is no absolute requirement to have it in your scenario, but it wouldnt hurt to create a new one and clean that up either
and its pretty easy to do.