Here we have the same battery only now it is oriented with the cells running bow to stern. As you can easily see the positive and negative battery plates are exposed and uncovered from the electrolyte. On these batteries spilling is not an issue due to the height and design but on the typical 12V "marine" group 24,27,31 etc. this can not only lead to exposed plates but also electrolyte leakage.
Exposing or uncovering of the positive and negative plates of a battery from the electrolyte, especially when under load or when being charged, is not good for the battery. Even if you are not spilling electrolyte you may be exposing the plates.
Seeing as most sailors are using the batteries to power house loads, while they are sailing, proper orientation of the cells can be that elusive piece of the puzzle that finally leads to longer battery life.
You may have invested thousands of dollars in solar, wind, regulators, alternators, desulphators, fancy chargers, new large gauge wire, bank fusing and more, but if your batteries are improperly installed you're still missing that last piece of the puzzle...