The main practical purpose of sideslipping is to get down the mountain without picking up speed.
Many learners are deprived of this skill because it is generally undervalued and has become even more neglected due to the complete domination of carving skis.
Wider skis are easier for sideslipping in a greater range of conditions. Parabolic skis have some trouble gripping on ice during a sideslip, but modern “double rocker” off-piste skis give a smooth and grippy sideslip in  just about every condition, including ice. I currently highly recommend the K2 Kung Fujas as a great all mountain ski.

The skis are kept on edge by the lateral stiffness of the ski boot shafts. Skis over 100mm wide underfoot begin to create problems on hard snow due to the extra leverage from the edge through the shaft of the boot. Those skis can be very unpleasant on-piste and it’s one limit of how an “all round” ski can be defined. Anything wider than 100mm is not “all round”. 

In ski teaching the sideslip serves specifically for developing fall-line skiing. Fall-line skiing is where the skier’s body travels directly downhill and not across the hill. This would apply to bumps, steep off-piste such as couloirs and deep powder snow. Slalom is not “fall-line” skiing.
The skier should be able to sideslip on either ski or both at the same time. It’s normal to start off with both skis on the snow, skis parallel and with the majority of weight on the lower ski just to get a feel for it. The stance is normally quite narrow to prevent the uphill ski from catching the lower edge. Most beginners have trouble keeping the skis close together.

It should be noticed that only the uphill edges of the skis are in contact with the snow so the downhill edges are in the air. If the fronts of the skis are “pulled” downhill then there will be no resistance from the downhill edges and so the skier goes into a forward diagonal sideslip. Likewise if the tail are pulled downwards during the sideslip then it turns into a backwards diagonal sideslip. Being able to alternate between straight down the fall-line and the two diagonals is a precursor to the skill necessary for pivoting in the fall-line.

Both feet need to be rolled onto their inside edges inside the ski boots. This is not only an issue of “edge control” but of body management. Rolling the feet using the subtaler joints beneath the ankles and pulling inwards with the adductor muscles on both legs ensures the best options for control. Pulling both legs inwards keeps a narrow stance and when the skier practices sideslipping on one leg it ensures the that centre of mass can easily be placed above the appropriate hip joint.

Sideslipping needs to be practiced on the uphill ski only, with the foot rolled onto its downhill edge inside the ski boot – ski still on its uphill edge. This is preparation for learning to pivot.