gui_m_p
A vesting period allows us to manage our resources more effectively, as its spread out over time. If we did it instantaneously it would be similar to a rage-quit and use all the resources at once.
The most common example of this you see in banks today is the insurance a bank gives you to prevent bank runs. Because insurance is in place, many people are content with leaving their assets their, securing a larger value than what actually can be secured.