It is best to start this question with a definition of eyewitness. An eyewitness is someone who has direct firsthand knowledge of an event in some way with one of their senses, usually sight. For the most part, eyewitnesses are considered more reliable than circumstantial evidence, and so from this perspective, eyewitness evidence is important.
Eyewitnesses can be subpoenaed by the law to testify. This underlines once again the weight an eyewitness can have.
With this said, we need to be careful, because eyewitnesses can be either unreliable or their credibility can he called into question. There have been instances, where eyewitnesses have been unreliable. Some experiments have even been done on this topic. Also it is very possible for an eyewitness to be not credible due to personal biases. In the end, the testimony of eyewitnesses also needs to be weighed.