How does wearing the suit affect the speaker's identity?
When the speaker puts on the suit, he is able to leave his house and enter the world. This transition allows him to encounter other people and things, and upon encountering them, his consciousness is transformed. This new understanding gives him material for his poetry, and so makes him a poet. The speaker implies that none of this would be possible if he remained naked at home.
Why does the speaker "forget" his suit?
The speaker forgets his suit when he feels that it has become a part of him. The poem is an ode to the suit, and he does spend time noticing its qualities and paying it respect as a separate individual. These are moments when the suit is personified and takes on a life of its own. It acts as a companion to the speaker, living a parallel life. But by the end of the poem, they have become "one being," so the speaker forgets their difference.