Two founders split the equity evenly and start work. One leaves after five months. Without vesting, they leave holding half the company, and the person still working owns half of a business the departed founder will benefit from indefinitely.
Vesting means shares are earned over time, commonly four years with a one year cliff. Leave before the cliff and you keep nothing. Leave after two years and you keep half.
It reads as distrust when you are drafting it and as common sense the first time a founder departs. Put it in at formation, when nobody has a reason to object.