It is important when two different floor coverings are at different heights, that the transition is smooth so as to not cause people to stumble, or wheels to catch. In the photo you can see the difference between two thresholds that both rise up the same height, but one being very narrow, creates a dangerous bump. The wider one provides a gentle slope for wheels and feet alike.
If the two floor coverings are close to the same colour, yet at different heights, it is important that the threshold itself is a contrasting colour to warn people that there is a change in height. Without the noticeable colour difference, people will stumble.