Why the Router Exists

Key takeaway: Uniswap V2 deliberately separates the immutable, security-critical core (Pair, Factory) from the user-facing periphery (Router, Library). Calling pair.swap() directly forces users to compute fee-adjusted output amounts, sort token addresses, transfer tokens to the pair before the call, and accept whatever price the pool gives them. The Router solves all of these: slippage protection via amountOutMin, deadline enforcement against stale execution, multi-hop path routing, and WETH wrapping for ETH paths. Periphery can be replaced; core cannot.

The Problem with Raw Pair Calls

You have built the core protocol. UniswapV2Pair exposes mint(), burn(), and swap(). These functions work. But they are dangerously low level.

If a user called pair.swap() directly, they would need to:

1. Calculate the exact output amount themselves, including the 0.3% fee math.
2. Transfer the input tokens to the pair address before calling swap.
3. Sort the two token addresses to figure out which is token0 and which is token1.
4. Set the correct amount0Out/amount1Out based on the swap direction.
5. Hope that no one front-runs their transaction and moves the price.

One mistake in any of these steps means lost funds. No safety net. No refund.