If you actually try putting ships on land you will realize they suck. Without the ability to move, their usefulness is quite limited. Ships are also quite expensive across the board, meaning that it makes more sense to just build a Cerberus if you want fixed artillery on land. A Cerberus costs roughly 3 Envoys or so, and about half of a Shogun.
In fact, its such a bad strategy that its considered trolling your own team to do this.
That being said, if you have won sea completely, and your ships can't carry out a meaningful bombardment campaign against the enemy from their current position, you might as well move them onto land, as otherwise they will just sit around doing nothing for the rest of the game. Reclaiming them would only give you back about half the metal.