For the Resting Period via the Dark Retreat, the key is not really vegetarian food/diet (which certainly has its benefits) - but depriving the senses of stimuli that keep it busy and distracted - sights, smells, light, food etc., so that energy that is wasted through the nine “holes” of the body are sealed and used to effect internal alchemy.
Food of any kind is restricted because digestion/elimination/assimilation consumes a lot of energy in itself and drags consciousness down while it happens. Again, this is a very advanced practice and not for people who have not put in decades of work achieving a certain level of energetic and emotional clarity.
The key aspect is sensory deprivement followed by sensory withdrawal, accomplished here forcibly (Pratyahara of Yoga) - forcing the consciousness to re-direct itself towards the Self and begin to inch towards Dharana (Effortless Concentration) and Samadhi (Absorptive state of Non-duality). When this happens, “awareness of this or that” changes to “Awareness”, or pure cognition, which can be life changing.
Risks come along the way - too much energy generated, if not put to right use, can cause schizophrenic syptoms, inner demons creep out which can attract outer demons, one may open oneself up to various other dimensions some of which are not very safe etc. Which is why one uses preparation, protective rites, mantras etc. to aid this quest of Pure Awareness.
Do people absolutely need this? No! Gradual practices, though slower, are safer for most people.