>>> How so?  AFAICS, it's the exact same kind of output, except that it 
>>> gets truncated.  And it's (obviously?) better to see the context of 
>>> the pattern you are searching for, instead of the first characters of 
>>> the lines on which the pattern is found, in which the pattern might 
>>> not be present.
>> 
>> Since Grep doesn't return the column number of the match, we get it 
>> from parsing the string again. And if the string is now modified to be 
>> truncated from both sides, the column number will become wrong.
>
> I did not understand that you need the column number of the match. 
> That could perhaps become a feature request for GNU grep: with -o and 
> -n, also print the column number of the first character.
>

I wrote too fast.  In fact you can get the column number with GNU grep 
without parsing the original line:

grep -nb -oE '.{0,100}PATTERN.{0,100}'