Unagi has the highest Crock Pot priority in the vanilla game (20). This means that placing an Eel and Lichen into any recipe will always result in Unagi.
It restores less Hunger than the Eel and Lichen combined, but heals 20 Health and 5 Sanity, so it can be useful when Health is low. However, it spoils in 10 days (which is also affected by the Lichen's fast spoil time), so it is recommended to only cook it when needed or put it in an Ice Box.
Since Unagi has no filler restrictions, it can be used to regain Health and Sanity quickly in the Ruins by using either the naturally spawning Blue Caps or dropped Monster Meat as filler.