Whats the point of doing lactic training/race spesific training during base training?

Lyddiard system is more complicated than most realize. He never leaves speed any week of the year.
It is super tough for some, easy for others - My immune system suffers big time with Lyddiard - but thrives on Multi Pace. But multi pace my O2 fitness is not as good as Lyddiard.
I’m trying more of a Weekend long runs and weekdays Multipace now. Hopefully it will enjoy