Why insiders don't get their tokens all at once
Team members, early investors, and advisors are usually promised a token allocation well before the public ever gets a chance to buy or claim any. If that allocation were handed over in full the moment the token launches, those insiders would have every incentive to sell as soon as the market allows it, often while hype and price are at their highest right after launch, since that's typically the best price they'll ever get. That kind of selling from people who had privileged, low-cost access looks especially bad to the rest of the holder base, and it can flood the market with supply at exactly the moment new holders are deciding whether to trust the project.
Vesting addresses this by releasing insider allocations gradually over a defined period rather than all at once, often starting only after an initial cliff during which nothing is released at all. The effect is that insiders remain economically exposed to the token's price for a long stretch after launch: if they want their full allocation to be worth something, the project needs to still be healthy and the token still valuable months or years later, not just on launch day. This aligns their incentives with the protocol's long-term success rather than its short-term price spike, and it gives the rest of the holder base a visible, predictable schedule for when additional supply will enter the market, rather than an unpredictable dump risk hanging over every insider wallet.