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The center of the map can often be a death-trap in the early-mid game. Enemies will be out wide on both flanks fighting for position and could potentially shoot at ships in the center from multiple angles which is hard to protect from. Some ships are small or work especially well with island-based positioning, so they can play more middle-map positions with an expectation of safety or capability.
Ships that can excel on a flank will often be damage resistant ships which can be played open water, or ships that require being out at wider angles to make sure their shells can pen their enemies. The damage resistant (or avoidant) nature of a ship can help to win the trades on a flank and slow the enemy's advance, turn the advance, or win outright through attrition. As for the angle requirements, Italian cruisers are an example of a shipline that needs to have enough angle on their targets for the SAP shells to bite in.
Island based ships can also perform well on flanks, but may take some amount of open-water movement to be able to get to a far island and dig in. If the ship is spotted while in transition, it can mean risking heavy damage or long delays as the ship will have to sail out a distance before being able to continue heading to the far-position for dug-in cover.