To ensure complete wind protection. The ports on any shotgun microphone need to be covered as well as the tip. Wind will still get in through these ports (and depending on the make and model, some switches) on the microphone if left exposed.
The best Softie solution for the Sanken CS3 would be 18cm:
Description: 18cm Super-Softie (19/22)
Order Code: 033203
Web link: www.rycote.com/products/033203 UK