Data allows for more effective decision making, but data is only as useful as your ability to use it. Metrics allow you to step back from your ideas and time and ensure you are meeting the needs of users. Analytics focused on users' behaviour can be a good source for explaining why you made a decision.
Metrics are often not useful on their own. All metrics need to be situated in context and focused around outcomes (e.g. the number of users to the site isn’t useful, but if you know the size of your potential audience, then knowing how many people you are reaching vs. could be reaching is helpful.)
Try to stay away from vanity metrics. These are numbers that don’t tell you anything in isolation. (e.g. number of page views). They tell you nice/bad stuff but not what to do next or why it is happening.
Mix methods approach to evaluation can often help to fill in the gaps in your knowledge.