It helps get rid of bacteria and wild yeasts. The lack of cleanliness of equipment is considered by many people as the main cause of bacterial problem (one could argue that this is a pretext not to say "I have no clue why there is such a problem".)
Wash everything (carboy, vessel for the starter, rubber stopper, spoons, thermometer, hydrometer, funnel) with soap water, then rinse.
Soak in cold water with bleach for a few hours. Rinse with hot water until no bleach odor remains (bleach would kill yeasts.)
Some products exist which do not need rinsing. Some people use their dishwasher.
It is to banish bacteria from the must (water-honey mixture, not fermented yet or fermenting) that a starter is prepared. Which means that yeasts are not directly dropped into the must, they are first allowed to respire in order to store energy and multiply before starting fermentation [H10]. Yeasts can multiply and they will produce alcohol faster, which kills wild yeasts [H9]. Do not add any sulfite, as it would kill the yeasts.
Boil 1.5 qt of water for 10-15 min and then add ½ pound of honey and let it cool down. When the temperature is as low as 100°F, pour in a 2 qt vessel and add 2 ½ tsp of nitrogen-containing nutrients (use lower quantities with honey containing more honey than average and higher quantities for honeys lower than average) and ½ tsp energizer (vitamins). Mix well and add yeast (quantity for 5 gallons).
Dissolve some sulfite in hot water. Soak a cotton disk in it and dry the cotton squeezing it between your hands (wear gloves). Shake gently the yeasts and put the cotton on top of the vessel. This should prevent bugs and dust from entering the starter, while allowing air to come in and provide yeasts with oxygen. SO2 should repel bacteria. Let sit for 30 min, maintaining the temperature around 100°F (place the vessel in warm water) without stirring (this would damage cell walls which are still weak at that time [Lalvin]) then stir again. Agitate from time to time until the fermentation is vigorous. This is short with dry yeasts but can take days which liquid forms which are a bit old, in such a case see maker's directions). Use at the peak.