Silent letters in names usually come from one of two places. In Irish and Scots Gaelic, the letters bh, mh and dh soften to a V or W sound, or drop out entirely. In English, silent letters are often leftovers from older pronunciations or imported spellings.
Examples worth knowing
Siobhan contains a silent bh (pronounced Shi-vawn). Aoife has a silent f at the end (Ee-fa). Sinead has a silent e and a softened d (Shi-nayd). Sean is simply pronounced Shawn. Once you know the rule, a whole family of names opens up.
Why silent letters survive
The spellings preserve cultural and linguistic heritage. Simplifying them would strip the name of its roots. For parents willing to teach the pronunciation, silent-letter names carry real depth.