When you sign a transaction it goes to the mempool, visible to anyone running a full-node. Producers assemble blocks from it, generally sorting by fee per unit of size, so low-fee transactions sit until demand falls.
The mempool being public is a trading problem, not just a plumbing detail. Bots watch it and can profit from your pending swap through mev tactics such as the sandwich-attack. Routing through a private-mempool avoids that exposure.
Example: your swap sits with a 5 gwei priority-fee while the market clears at 40 gwei. It will not confirm until congestion eases; you can bump it with replace-by-fee or cancel by replacing it with a zero-value transaction at the same nonce.