Thickened liquids are often easier for dysphagic patients to swallow than thin liquids because they are more viscous and have a much slower transit time during the process of swallowing. Also, the thicker consistency of the liquid reduces the likelihood of it silently seeping into the lungs, which lessens the chance of aspiration pneumonia developing.
Think about this. If you pour a glass of water or coffee onto the floor, it will make contact with the floor in a split second. However, if you try the same thing with a thicker substance such as maple syrup or honey, the transit time that elapses until it reaches the floor will be slightly longer.