Then they switch to being Betty Grammar. Books often reflect these changes as collections from a particular period. How to know when you have changed as a poet may be, well, mystical. A simpler way to tell when the book has come is reading yourself critically. Some of the poems should be published in journals, for one thing.
Poets assume editors have poor taste, but sometimes they do know something.
Maybe, for example, your Blue Period poems will embarrass you after the smoke has cleared. Good editors can tell you that. And they can, by publishing the poems, develop an audience for your work. A little string of publications shows a publisher your poetry has an audience. Few things in the world are more forlorn than books of poetry sitting on bookstore shelves never being bought.