When do I need to design an interface?
New product development.
For new projects, lots of attention is paid to market analytics, analysis and study of the target audience, its needs and tasks. An accurate understanding of future users and the ability to predict behavior are prerequisites for creating a user-friendly interface. Ideas and functionality are visualized in a prototype, an instrument where the visions of the entire team and the owners of the product are synchronized.
A high-fi prototype is used for testing with potential users. Getting feedback from potential customers, you can quickly make edits by optimizing the functionality. This will help to save development resources in the future, avoiding "rework". The finished prototype of the interface is the basic platform on which marketers are oriented (evaluation of SEO promotion, etc.) and developers (understanding the functionality that needs to be developed).
Redesign of the existing project.
When you redesign an existing site, service or application, you need to keep in mind that there are users who are already used to the interface, so the task is not only to improve the existing interface, but also not to lose regular users by drastically changing the interface.
Analytics of the current site is the key source of information that will give answers to the question, what exactly needs to be improved and how. Such tools are used in the redesign: analysis of statisticall systems (Google Analytics), qualitative and quantitative research of the audience, analysis of user behavior, subjective satisfaction metrics, etc. In the process of analysis, hypothesis (assumptions that can be improved) are collected and the current audience of the site is involved in verifying the the hypotheses with the help of surveys, usability testing, scenario modeling, etc.