When you see someone doing this, it's because they're taking a shortcut, usually without even realizing that they're doing it. On the surface of the earth, where acceleration due to gravity is roughly 9.81m/s^2 no matter where you are (it DOES change a bit but not by a huge amount), then we can use the force an object applies to the ground to determine what its mass is. In other words, we can treat a unit of force like a unit of mass. Doing so will get you yelled at by physicists, so don't do this in any scientific or engineering environment.