Patterns defined by any theme are always locked because if anyone were to alter them, when the theme updates, the patterns would revert to their default state and any user changes would be lost. By copying and modifying the copy, you protect the modified pattern from being reverted during updates.
When a pattern is synced, any changes to one pattern affects all instances of that pattern anywhere on the site. Non-synced patterns exist as their own entity. Changing one only affects that one instance, not any others.