There are two parts to your question which I think are important to address: the importance of role modelling in general; and the importance of ethnicity/gender/sexual identity in particular.

First, role models are important because they show you what is possible for you to do. It's much easier to copy what someone has done before than it is to experiment yourself. For example, imagine the different types of work you can do to make money. Would you imagine that you can make money playing video games without seeing other people making money? Watching e-sports or streaming informs you that these are careers where some people are successful at earning a living. Role models are proofs of concept in ways to live your life. Without them, you would have to experiment yourself and make a bunch of mistakes on your way to living that kind of life.

Secondly, ethnicity/gender/sexual identity affect both your path and your destination. At least historically, people in the U.S. have been told that they can't do things because they are who they are, whether through explicit discrimination or by a prejudiced view of their capabilities. Seeing someone succeed in a role with a shared characteristic shows that the characteristic doesn't prevent you from achieving that role.

Furthermore, the way that these role models fulfill their role can be different than "normal". For one, their success may show that you don't need to hide aspects of yourself to protect yourself from others. Being gay is different from being out and known as gay. If it's easier to be out in some professions than others, then you may avoid being e.g. a teacher because the cost of remaining closeted may make it not worth doing. The role models aren't just for you to emulate, they are also comrades in social acceptance.

Finally, I want to acknowledge that these aspects are never the entirety of a person's identity. They are specific aspects which sometimes have zero impact, sometimes a great deal of impact, and very rarely total impact on a situation. The important of ethnicity/gender/sexual identity will vary based on the time and place. Ethnicity, gender and sexual identity are important parts of role models in the U.S. because those have been important parts of identity in the U.S. which have major influence on what people can do. Maybe their importance will wane with time, and other aspects of your identity will rise to prominence.

If you want to do some reading on this, the search term you're probably looking for is "representation matters."