We have only one Apple ID. Do I need family sharing?
Is family sharing necessary when only my wife and I use the same Apple ID?
No, it is for sharing between IDs.
What is Family Sharing? - https://support.apple.com/HT201060 - "Family Sharing lets you and up to five other family members [Apple IDs] share access"
However, you may wish to read this: Apple ID Frequently Asked Questions - https://appleid.apple.com/faq