Reversed-phase HPLC separates the components of a peptide sample and integrates their peak areas, producing the familiar percentage purity figure. It is a powerful quantitative tool, but it measures what is in the vial relative to everything else that elutes - it cannot by itself prove which compound a peak belongs to.
Mass spectrometry answers that second question. The observed molecular ion is compared against the theoretical mass of the target sequence; a match within tolerance is strong evidence of identity. Deletion sequences and side products that co-elute with the main peak can otherwise be silently counted as 'purity'.
That is why the marketplace requires both results on file, on the same batch, before a listing activates. A vendor that can produce HPLC and MS data for the exact lot they ship is a vendor you can build a reproducibility story on.