Stage separation by design is typically slightly propulsive. The goal however is not to add a significant amount of momentum to the part of the vehicle that still matters. It is instead to ensure that the part of the vehicle that still matters doesn't collide with the part of the vehicle that no longer matters. Stage separation is a risky event. Several launches have failed due to collisions between recently separated segments.
Avoiding these risky stage separation events is one of the reasons why single stage to orbit (SSTO) has long been a very desirable goal. SSTO would be a highly disruptive technology. It might however be an unachievable goal, at least using current or near future technologies. Multiple decades of research have gone into this concept with no success (not even a promise of success) to date.