Is it necessary that a piano tuner know how to get around on the piano? No.  Piano tuners can competantly tune an instrument one note at a time.  HOWEVER, that said, it is an INCREDIBLE asset to know how to play to CHECK the tuning.  My background is in performance and teaching.  I started tuning only about 10 years ago.  When i first started tuning i would then play a favourite song and say to myself “BLECH… this is TERRIBLE!” hahaaa.  Then off i would go fixing what sounded ‘out’.  For a brief period in college i studied ancient Greek language.  The first 3 rules of interpretation are… context, context, context.  Similarly when tuning, the context of a note within a chord reveal pitch.  I’ve spoken about how some notes on certain pianos don’t sound ‘right’ and you can alter them to sound more…mmmm soothing instead of jarring or clashing.  And the way in which you tell if a piano sounds in tune is from playing a song, an arpeggio, a chord or melody.  So is it necessary to play if you’re a tuner? Absolutely not.  Anyone can play 2 notes at a time.  But is imperative to check the tuning?  Absolutely.