from https://www.viqtorysports.com/split-field-coverage-in-football/
Split field coverage allows coaches to be multiple in their play call. Instead of telling the entire field they’re playing cover 2 or cover 4, they can now split it and play different coverages on each side. This allows the coach to gameplan to certain formations and not be so predictable with every coverage.
Split field coverages also put defenses in a better position to be multiple based on TEs or WR’s in the game.
Last, the split field coverage look allows the two high safeties to get into the run fit. Often they will play at 10-12 yards and are the team’s best tacklers. This allows coaches to get two extra players into the run fit to help on the outside runs.