When I was little(r), almost all small portable radios used 9-volt
batteries. There were some, of course, that used AA's (or even 1 AA; I had
an am portable with 1 AA cell that we snuck into school when
something-or-other REALLY important to schoolboys was going on), but now I
don't have & don't know of any radio that uses 9V batteries.

Why were they needed in the first place? The 9-volt package has a lot more
package and a lot less mWatthrs in it. Was it because of the voltage - did
the existing solid-state tech not work so well at 3V?