Hydrating minerals, even by tiny fractions of a percent changes their properties markedly, most particularly it lowers the melting point of the Mantle and allows small amounts of melt to develop at depth and rise towards the surface. This could help explain some of the very unusual magmas seen in kimberlite pipes (where we get diamonds) and under the African Rift Valley.
Until now it's been thought that water was mostly limited to comparatively shallow depths (c. 600km or shallower) and largely the result of ocean crust becoming hydrated shortly after eruption at ocean ridges. When the ocean crust is subducted into the Mantle, a combination of heat and pressure drives water out of the recrystallising rocks and into a wedge of Mantle above the sinking ocean plate. There it causes melting of silica rich magma and is directly responsible for the extremely violent volcanism seen around the Pacific.