I haven't been to anywhere this summer.

为何这里需要介词to，而go somewhere 就无需。

be to 用于目前完成时have/has been to = have/has visited. 需要后接名词作宾语。anywhere在这里不是副词，而是代词=anyplace, 作be to 的宾语。gosomewhere中，somewhere是副词，作不及物动词的状语。