From a purely academic point of view, music in a pre-school classroom is very important to the development of the pre-school child.  Listening to music, playing music games, and singing helps children use various parts of their developing brains, at the same time.  It stimulates  their language, mathematical, and motor development.

From a purely non-academic point of view, music in the classroom is  fun. 

We sing to express ourselves, it makes us happy, and can be a good distraction (or in classroom terms, redirection).
  

There are many practical reasons to sing in a pre-school classroom.
We sing at transition times.
When gathering in the morning for our morning meeting we sing: 

(to Farmer in the Dell)

It's time to clean up now,

 It's time to clean up now,

heigh-ho it's time to go, it's time to clean up now.

When the children hear this, they know it is time to gather. 
Even when at times I fumble for the tune, by singing the initial notes of the song,  the children redirect their  attention and  prepare for cleaning up or choosing their name card.

Similarly at the end of play time, we sing:

(to Mary had a Little Lamb)

Our friends will meet at the tree, at the tree, at the tree

Our friends will meet at the tree, 

At the tree we'll meet!

When we are getting close to the classroom we sing

Singing is an excellent teaching tool.  
Songs are catchy,  and offer a fun way to teach basic skills, body control and social skills.

Singing increases everyday and thematic vocabulary.
Rather than providing children with a definition or a list of attributes about a topic, a song provides a fun and easy way to remember the facts and information.

 Songs get the children involved and active. 

In our  preschool classroom we sing our prayers and our blessings.

Songs are joyful, and none more so then when sung with young joyful voices.