I know some people that, to this day, will not set up their hookahs without ice in the base. Personally, I stick to room temperature tap water but every once in a while you want to take things down a few degrees and enjoy a cold smoke.
The most obvious reason to add ice to the vase of a hookah is to chill the water and make the smoke colder for a while. The ice will melt and the water will eventually warm up but while it last the effect can be nice. Especially on a hot summer day with Social Smoke Absolute Zero in the bowl and crushed up Altoids in the base water. It’s like breathing liquid nitrogen.
The other reason that people put ice in their hookahs is to reduce the harshness of the smoke. If the bowl is packed properly and you are using the right amount of heat you should never have to hide harsh smoke behind ice. If you are experiencing harsh sessions I suggest you try to perfect your packing techniques and heat management before you add any ice to your vase. Adding ice before you get everything perfect is just putting a bandage on the problem rather than fixing it at the source. Also, the colder the smoke the more muted the flavor is going to be. I don’t suggest using ice with subtle flavors. Stick to mints.
Be careful when adding ice to the vase as well. Cold glass is more likely to crack than room temperature. If you do add Ice I suggest you add a good amount of water and then slowly add the ice cubes. This acts as a buffer. Not only from the cold but from the impact of the ice cubes hitting the glass as well. If you can, I suggest using crushed ice too. If you use mostly crushed ice and a little water it kind of acts like a diffuser.