Why is classpath definition necessary in java and javac?

Question

Java

Good day!
I am writing an application that uses third-party libraries. I was perplexed when I saw the classpath option for java and javac in help .
After not very successful attempts to learn about this circumstance in English google a little more than " Classpath is a parameter in the Java Virtual Machine or the Java compiler that specifies the location of user-defined classes and packages. ", came to you.
I understand why the classpath needs to be defined. I don't see why it might (or might not) need to be done at both compile time and run time. Hope you can shed some light on me.
Thank you!