Hi Fritz!  Well, I'm sure someone will have a more scientific
explanation, but one purpose is that the pitch is lowered by cutting
out the middle, thus one can produce a lower tone with a shorter bar
(within limits).  I think it also affects the 'ring time' and note
quality of the bar.  Conversely, a bar's pitch is _raised_ by nibbling
away the ends.

The same can be done with metal bars but, since you're starting with
thinner material, the 'adjusting range' in the middle isn't as great.

Most metal glockenspiel bars will have grinding marks in the middle of
the bottom surface, a result of the tuning process.  I have a 30-note
set of Deagan glock bars from a theatre organ, and about 1/2 of them
bear grinding marks on their lower surfaces.

If you ever attempt to alter the pitch of a bar, _make sure_ that the
bar has returned to room-temperature before you check the pitch.  The
heat produced by the friction of 'grinder tuning' greatly affects the
pitch until it returns to a stable temperature.

Nickel-plating a bar lowers the pitch by about 12-cents, I think.

Hope this helps. :)

Ron Yost, Paso Robles, Calif.