Why do some pianists do "vibrato"?

It's actually not entirely daft, but in something like 98% of instances you see, it is utterly so -- and 'daft' includes affected, mannered, deluded, and so forth... :-)

However -- yes, the big 'but' <g> -- certain colours can only be achieved from a starting point of extreme hand tension -- almost every one of these effects concerns dynamics of 'pp' and below, and never any higher than 'subito (più) piano' -- after which an almost imperceptible 'vibrato' of the hand supported on the key-bed helps rapidly to dissipate the momentary high tension needed for the colour production in question, while clinging to the key surface maintains position. I.e. the 'vibrato' is about what happens *next*, not the sound you might notice it seemingly being applied to (in vain).

(Example: I will do this immediately after executing a high preparatory hand tension, true 'ppp' but still-projecting top Ab (it must whisper *and* carry) that closes the 2nd mvt of Chopin's second concerto, to free my hand, without distracting by any other, plainly visible movement, from a. the moment itself, and b. without giving away to the audience when I will slip into the 3rd movt 'out of nowhere', co-ordinating ensemble by a covert flick of the eye between me & the conductor only. I physically do need to execute that barely perceptible ('front row stalls only') 'vibrato' or my hand tension will be all over the shop and my arm weight miscentred at the start to what follows, and I absolutely do not want to allow the audience *any* sense of 'letting go' between the second and third movements.)

Some instances can be born from composer indicated necessity. Liszt, in his (in)famous, terrifying, ever more hushed, static B-major chord sequence that closes the b-minor Sonata, marks a crescendo in one chord *after* it has been sounded. Skilled pedal control causes the necessary 'bloom' in the residual sound already struck to become apparent, and pointing that moment up with the merest but now intentionally visible hint of an 'on-key' vibrato movement -- deploying pure theatrical gesture, in short, combined with physical need because of colour -- can make the difference between an audience 'getting' that (logically) perverse but utterly crucial 'crescendo' of Liszt's that makes that closing sequence the extraordinary moment that it is or certainly *can* be, or not...

Lastly, my friend Switch ♪♫ has, in jest, more than a grain of truth by the tail that, truly involuntarily, occasionally that 'vibrato' movement can happen as a subconscious wish-led action in imitation of string players. I know I have done it very occasionally and don't feel the slightest need even to think of apologising for that. :-))

All the best,

Not on a piano, but there are plenty of keyboards that have aftertouch sensors that allow for vibrato. Early ones worked from wiggling side to side, later ones use pressure.

... really early ones were theater organs that had "second touch"

--

"Second touch is a feature of Wurlitzer manuals and pedals that activates a second registration when the key is pushed with a bit more pressure -- 1/8 inch past the first touch depressed-key position (illustrated above). Accompaniment and melody line of a chord can be played on one manual: accompaniment on first-touch with normal key pressure, melody line with extra key pressure on one key to engage second-touch. "

http://www.shomler.com/calsj/secondtouch.htm

hahaha, ive never noticed this, but i am a violinist and whenever i play guitar hero, i add in some vibrato

because deep down everybody wants to be a violinist. we rule the world with an iron fist!

the guitar hero thing (eh) - but you do use vibrato on guitar.

why can't we have musical video games that actually teach you something....