для чего нужен mesh collider в unity3d

There are some limitations when using the Mesh Collider. Usually, two Mesh Colliders cannot collide with each other. All Mesh Colliders can collide with any primitive Collider. If your mesh is marked as Convex, then it can collide with other Mesh Colliders.

Collision meshes use backface culling. If an object collides with a mesh that will be backface culled graphically it will also not collide with it physically.

The Mesh Collider builds its collision representation from the attached to the GameObject, and reads the properties of the attached to set its position and scale correctly.

ConvexIf enabled, this Mesh Collider will collide with other Mesh Colliders. Convex Mesh Colliders are limited to 255 triangles.

Smooth Sphere CollisionsWhen this is enabled, collision mesh normals are smoothed. You should enable this on smooth surfaces eg. rolling terrain without hard edges to make sphere rolling smoother.

MaterialReference to the that determines how this Collider interacts with others.

Is TriggerIf enabled, this Collider is used for triggering events, and is ignored by the physics engine.

The Mesh Collider takes a and builds its Collider based on that mesh. It is far more accurate for collision detection than using primitives for complicated meshes. Mesh Colliders that are marked as Convex can collide with other Mesh Colliders.