As World War II progressed into the Pacific, it was clear that the US military would need to be able to operate farther to the east. However, the distance involved meant that the US would need a series of bases close enough to the Pacific Theater that could support these operations.
The Marianas Islands provided an area that would put US operations within range of the Philippines, Japan, and other regions of the Pacific. However, as the Marianas were heavily defended by the Japanese, the US would need to use a series of land-based bombers to help break up the defenses there.
By capturing the island of Tarawa, the US would have a better location to begin it's assaults on the Marianas. However, the Japanese were also well aware of the strategic value of this island, and had already heavily fortified it's defenses.