Here's why:

* Current Direction: A galvanometer is designed to be sensitive to the direction of current flow. It uses a coil suspended in a magnetic field. When current flows through the coil, it experiences a force that causes the coil to rotate. The direction of rotation depends on the direction of the current.

* Measurement: While it doesn't directly measure the magnitude of current, the direction of the needle's deflection indicates the direction of the current flow.

Note: Modern ammeters, which measure current magnitude, often incorporate a galvanometer within their design.