The reasoning was that those islands could simply be cut off from their supply chains (leading to their eventual capitulation) rather than needing to be overwhelmed by superior force, thus speeding up progress and reducing losses of troops and materiel.
As the 20th century dawned, the U.S. had several interests in the western Pacific to defend; namely, access to the Chinese market and its colonies – the Philippines and Guam – which the U.S. had gained as a result of the 1898 Spanish–American War.
[7] After World War I, the Treaty of Versailles gave Japan a mandate over former German colonies in the western Pacific—specifically, the Mariana, Marshall, and the Caroline Islands.
This is the very opposite of what is termed "island hopping" which is the gradual pushing back of the enemy by direct frontal pressure with the consequent heavy casualties which will certainly be involved.
[13]Leapfrogging allowed the United States forces to reach Japan quickly and not expend the time, manpower, and supplies to capture every Japanese-held island on the way.
They cultivated gardens using seeds and equipment imported by aircraft and submarines and worked with local labor, and remained strong and well-organized.
When MacArthur returned to the Philippines, he abandoned leapfrogging in favor of a policy of the complete destruction of the Japanese forces, mainly based on a political imperative of liberating the Filipinos.