Decentralization and the ability to provide liquidity are the two main advantages -- instead of a single signer that can run off with funds at any moment (Sideshift), you have a incentivized quorum of multisig holders who are strongly incentivized financially to act in the best interest of everyone.

Decentralization also makes Serai more resistant to government pressure or regulation, and will prevent geo-blocking etc. that is seen with Sideshift. It's really a middle-ground between CEXs and atomic swaps with much of the advantages of both, with lessened tradeoffs of both.

The ability to provide liquidity enables yield on native assets, but that obviously comes with many tradeoffs, namely (the poorly named) impermanent loss and custody risks (even if minimized, you do give up custody to LP).

While atomic swaps are the ideal "hard-core" way to swap between them, liquidity and UX friction will always be worse with a maker-taker model that is p2p than in a liquidity pool model. I do think both are necessary, but for most people Serai will be a better tool than atomic swaps.