As per the linked help article the ampersand defines the letter that will select the field when the Alt key is also pressed. Eg: &Year means pressing Alt + Y will select the Year field, when the Tag Panel is focused, since the & is placed before Y.
Removing the ampersands simply removes the Alt hotkeys.
As for whey those fields are called 'user defined' it's since the default fields are now customizable. Until the more recent Mp3Tag versions the default fields couldn't be customized. All of them now are (which is nice to have) including the default Alt hotkeys which are now exposed to the user.