why does a public defender waive court apperance ?

If you are stating the PD waived a case to a further court, that is not that unusual of a practice. Many times an attorney or PD will waive a case from a preliminary hearing, which means he did not choose to contest the matter at that particular hearing. Sometimes it is done because the lawyer or PD did it in exchange for some of the charges to be dropped, sometimes it is done in exchange for a lower bond, sometimes it is done because there is no defense and to litigate a preliminary hearing is a useless effort. You need to ask the PD. Don't call me, I do not know VA law. Ask the PD or call a local VA lawyer.