Fully parasitic plants or holoparasitic plants don’t photosynthesise, so depend on their host for all of their nutrition. The parasitic plants don’t benefit the host plant, and can even cause detrimental damage to them.
As well as fully parasitic plants, there are hemi-parasites, which are partially parasitic. These types still photosynthesise, but they get their water and nutrients from the roots of the host. However, hemi-parasites don’t need a host to survive, but if they are in an area that’s low in nutrients, they will thrive and grow faster by having a host.