Why Version?

You need a versioning strategy because your API will change. The question is not if but when. A good versioning strategy lets you:

Change Type	Breaking?	Explanation
Add optional request field	No	Old clients don't send it, new ones can
Add field to response	No	Old clients ignore unknown fields
Remove field from response	Yes	Clients parsing that field will break
Rename a field	Yes	Same as remove + add -- old clients break
Change field type (string → int)	Yes	Deserialization will fail
Make optional field required	Yes	Old clients don't send it
Change URL structure	Yes	All bookmarked URLs break
Change error format	Yes	Error handling logic breaks
Change auth mechanism	Yes	All clients need new credentials
Add new endpoint	No	Old clients don't call it
Add new enum value	Maybe	Depends on whether clients validate enums
Tighten validation (reject previously valid input)	Yes	Previously working requests fail
Loosen validation (accept previously invalid input)	No	Old clients never sent that input anyway

Included with REST API Design Guide