We live in a world where we need to move quickly and iterate on our ideas as flexibly as possible. Building mockups strikes the ideal balance between true-life representation of the end product and ease of modification.

Mockups are useful both for the creative phase of the project - for instance when you're trying to figure out your user flows or the proper visual hierarchy - and the production phase when they will represent the target product. Making mockups a part of your creative and development process allows you to quickly and easily ideate.