Aggregators simulate many routes, then split the order so the marginal price is equalised across pools. For anything beyond a small size this genuinely beats trading one pool, because price-impact rises non-linearly with size on each.
Look past the headline quote. More hops means more gas and more contracts touched, so on an expensive chain the cheapest quote is not always the best net outcome. Some aggregators also route through professional market makers who fill off-chain, which is a different risk profile from a pure on-chain swap.
Aggregators also change what a token-approval means, since you are approving a router contract that can move your funds across protocols. Approve minimal amounts where possible and revoke afterwards, especially for routers you used once.
Related: price-impact, swap-route, dex, token-approval