Why is a minaret an important feature of a mosque?

Yes, call for prayer can be done from anywhere but you to have a fixed place from where you can call for prayer and that people can know about it so that they can congregate and pray together. Islam believes in unity, one brotherhood standing shoulder to shoulder no rich no poor, believes in one heart. 

Minarets are not very important but they serve as an identity of a mosque so that people will know that this is the mosque. That is it.