When using getchar(), why do you need to clear the buffer for the enter key press in the first place?

Question

I'm new to programming and learning C:

printf("What are your two initials?\n");
firstInit= getchar();
lastInit = getchar();

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printf("What are your two initials?\n");
firstInit= getchar();
lastInit = getchar();
getchar();

I know that in the first section of code, if I type in ZK as initials, the Z will be placed in firstInit, and the '\n\ represented by the enter key will be placed into lastInit. The remaining K would be stored in a subsequent getchar(). However, in the second section of code, the book I'm reading said that if I type in ZK as initials, the firstInit will hold the Z, and the lastInit will hold the K. I just thought that the buffer needed to be cleared after each press of the enter key.

Sorry if this post is not allowed, will delete if needed.