Explain how bias in a keystream can lead to attacks

[trackpick]

I've seen the argument before, so I don't particularly need convincing (though TBH I've forgotten the details), but it would be nice to give readers an intuition for a possible attack based on a keystream containing bias. Noting this while reading the section on attacks on RC4 but it could equally well be connected to the description of one-time pads I guess.