What are the benefits of solitary practice? I have heard about buddhist monks who undertake strictly solitary practice for 6 months where they practice entirely by themselves. They have no human contact and some of the other monks will come and bring them their food. The other monks will leave the food at the entrance and only when they leave the solitary monk will come out to take the food. 

I'm very interested in this way of practicing and would like to know what the benefits of such concentrated practice is and the fact that there is no human contact or contact with the outside world.
If one has the maturity, will and energy, this can be very good. Unfortunately, most lack that kind of will until they have progressed a good distance already. Bodhidharma, the Zen patriarch practiced this way for nine years.

Isolated practice can only cut us off from humans, when concentration deepens, it can often lead to encounters with non-humans :-) It's never possible to cut us off from all sentient existence.

There are dangers and rewards with everything. Reintegration with society can be difficult, if we are not mature and stable enough, some experiences in solitary meditation can be very scary and disturbing, especially if we choose to practice in places with spirits, like graveyards.

It's like free climbing without a rope, not for beginners.

p.s. if you have not already, read the biography of Ajahn Mun Bhuridatto Thera