Each component in an assembly initially has six degrees of freedom. It can move along X, Y, and Z axes (translational freedom) and it can rotate around the X, Y, and Z axes (rotational freedom).
Affect of Joint and Constrain on degrees of freedom
When you place a constraint between two pieces of geometry, you establish an approximate position and remove one or more degrees of freedom.
When you place a joint between two pieces of geometry, you explicitly define the degrees of freedom and location. For example, a Rigid joint removes all degrees of freedom, and is commonly used to position components that do not move.
Reason to keep degrees of freedom
It is usually a good idea to connect or constrain components so that their behavior is predictable when related components in an assembly change position. For example, representing the assembly in an open or closed position.