The most common meter used in poetry is iambic pentameter (penta=five). Poets choose to use this meter when writing poetry because it gives the poem a strong underlying structure as a formal writing device. Iambic pentameter can be rhymed or unrhymed. In the case of unrhymed it is called “Blank Verse.”
For iambic pentameter, a line consists of five feet. A metrical foot in poetry goes by the name “iamb.” An iamb typically consists of one unstressed syllable and then a stressed syllable. There are variations of iambs that can be used in a poem so as to break up what sometimes can be a monotonous beat to a poem, especially if it is a long one.
This monotony can hypnotize a reader who can end up concentrating on the regularity of the beat of the poem and end up not paying attention to the words and the message being conveyed. You can liken this to listening to a tune on the radio with a steady unvarying beat, and tapping your feet but not really paying attention to the words.
Variations to the unstressed/stressed paradigm include trochees, anapests, dactyls, and spondees. For example, an anapest consists of two unstressed syllables and one stressed syllable. Therefore, the poem’s beat and rhythm is altered, which makes for a pleasant variation for the poem’s reader.
Iambic pentameter does have a regularity to it, which gives a poem a more formal tone and sophistication. It moves a poem along nicely and systematically and imparts a musicality and rhythm to a poem. This is why it was and still is popular. In fact, literary journals exist that are devoted entirely to formal poetry writing as opposed to free verse.
The most common meter in the English language is iambic pentameter. With it's unstressed/stressed pattern of sounds, this meter sounds more like regular speech that other meters. That's why Shakespeare uses it so often in his plays and other poets, who want their poems to have structure but also sound somewhat realistic, use it.