What is the purpose of requiring a defendant appealing after a guilty or no contest plea to obtain a certificate of probable cause?

The purpose of the requirement that a defendant appealing after a guilty or no contest plea obtain a certificate of probable cause, as provided in section 1237.5 and former rule 31(d), was to discourage and weed out frivolous or vexatious appeals challenging convictions following guilty and nolo contendere pleas. [Citations.] The objective is to promote judicial economy by screening out wholly frivolous guilty [and nolo contendere] plea appeals before time and money is spent preparing the record and the briefs for consideration by the reviewing court. [Citations.] (People v. Panizzon (1996) 13 Cal.4th 68, 7576, 51 Cal.Rptr.2d 851, 913 P.2d 1061, bracketed text in original.)