Swap like swapping of blockchain e.g from ETH to EOS or to mainnet, it simply means the cryptocurrency is migrating from one blockchain to another (most times from a third party blockchain to their very own blockchain).
I just want to add, it can also be swapping within the same blockchain but different contract address.
Why they (the project/dev?) need to swap the token?
In addition to what Brainboss already said, some devs do this to fix an existing issue with their smart contract.
I've seen a project that did what you said that they swapping within the same blockchain but the different contract address will be their new contract address that will be used and it's on short notice. Brainboss did explain the disadvantage already and also right about that only the project's developer know the reason why they decide to swap the token just like the xDAC which is an erc20 token and they swap it to xDAC coins. Anyway, I just use it as an example and I am not affiliated with the said xDAC tokens or xDAC coins at all and only use as example purposes of a token that will be swapped if the developer decided to do it.