Illiquid tokens usually have no direct pair with what you want, so the swap hops through a common asset. Each hop pays that pool's fee and incurs its own price-impact, so a three-hop route through thin pools can cost several percent even when each leg looks cheap.
Routes also determine failure. Any hop that reverts kills the whole transaction and you pay gas for a failed-transaction, and a route through a token with a transfer tax will deliver less than the interface predicted unless the router accounts for it.
Reading the route before confirming is a cheap habit. If a swap routes through a token you have never heard of, or through a pool with a few thousand dollars in it, you are about to move the price of something illiquid with your own order.
Related: dex-aggregator, price-impact, liquidity-pool, failed-transaction