Here's why:
* Proper nouns are capitalized. Family names are proper nouns.
* Plurals of proper nouns are generally formed by adding 's'. "Branches" would be the plural of a common noun like "branch."
It's possible that "Branches" could be a family name with a unique origin or spelling, but it's highly unlikely. If you're trying to find information about a specific family, it's best to check other sources like a family history database or genealogy websites.