I can speak for Jaithon but this is standard across most languages who have like immutable and mutable variables.
There is def nothing stopping you from using var everywhere, but (let) lets the compiler like catch any mistakes you did not mean to do. In jaithon, let is whats known as shallow: it prevents (in ur example) names = ... but the refrenced list is still mutable. Currently jaithon doesnt have anything like a read-only parameter so just knowing list[str] doesnt tell u anything about if a function mutates the list.
There is def nothing stopping you from using var everywhere, but (let) lets the compiler like catch any mistakes you did not mean to do. In jaithon, let is whats known as shallow: it prevents (in ur example) names = ... but the refrenced list is still mutable. Currently jaithon doesnt have anything like a read-only parameter so just knowing list[str] doesnt tell u anything about if a function mutates the list.