Android Japan
It isn't unusual for a poet to leave their work untitled, but even the word, "untitled", becomes a title in itself. A poem needs a reference for publication, and in the event that a poet chooses to leave their work untitled, the first line is often used.
Why Title a Poem?
The cereal box metaphor aside, there are, without a doubt, more choices for titles than there are individual poems and collections. Some may argue that a title limits the poem, as it may become a barrier to "entering" it. Others may argue that a title expands the poem's scope or offers clues to unraveling its meaning. Still others will claim that there needs to be a way to distinguish one poem from another, even if that is to label them as with some Classical music, "Opus 11 in A minor for Cello and Piano".
On Titles for Haiku and Related Forms
Traditionally, haiku and other English correlative Japanese minimalist forms of poetry arent' titled. When multiple poets are collaborating on a "leap frog" (i.e., rengay) or other form of multi-player linked poetry, there is usually a title in order to thematically, or otherwise connect, the poems.

