Full on racially motivated abuse is obvious and easy to understand. When someone tells me to go back to my own country and so on everyone around me can easily condemn that behavior.
Microaggressions are much more subtle. When someone is dismissive or rude to you it's very hard to tell whether they were doing that because of your race, gender, or because they are just an unkind person. For each individual action, it's basically impossible to say whether it was racially motivated or not. But after repeating this situation again and again you noticed that on average people are treating you a little bit differently.
I don't think it's worth complaining or trying to correct microaggressions. But I do think it's a useful term to describe what happens and to share experiences with others.