Modern cement when cured is very hard so when cement based mortar is used to repoint a building it will cause - through the cycle of wetting, freezing and drying over the years - the stone to slowly erode at the repointed junction; leaving the cement proud. At times it can even cause the face of the stone to crack away.
I repoint buildings without cement, using a Hydraulic lime/sharp sand mix. This is softer, porous and breathable. Lime mortar in pointing is sacrificial; meaning that when the wall goes through the cycle of wetting, freezing and drying, the lime pointing will slowly weather away over the years. To repoint your building is far easier and cheaper than having to replace damaged stonework.
Rendering is similar to plastering. But for the outside of the building. For properties in an exposed position or areas subject to damp, Hydraulic lime can be added to the mix to make it more resistant to weathering.