These functions return parts of the principal, either the realm or a specific component. The returned string points to data inside the principal, so they are valid only as long as the principal exists.
The component argument to krb5_principal_get_comp_string is the component number to return, from zero to the total number of components minus one. If a the requested component number is out of range, NULL is returned.
These functions can be seen as a replacement for the krb5_princ_realm, krb5_princ_component and related macros, described as intermal in the MIT API specification. A difference is that these functions return strings, not krb5_data. A reason to return krb5_data was that it was believed that principal components could contain binary data, but this belief was unfounded, and it has been decided that principal components are infact UTF8, so its safe to use zero terminated strings.
Its generally not necessary to look at the components of a principal.