An important purpose of“sarking” (or roofing) felt, or it’s modern breathable equivalents, is to underline the roof tiles or slates and provide a secondary defence against rainwater penetration: if rain permeates between the tiles/slates it hits the felt and trickles down to the guttering at eaves level and is then discharged via the downpipes.
Over the years the felt will decay. It takes a long time. On occasion, relatively localised areas deteriorate , tear of otherwise fall away as has happened here.
The advice is to patch up this fault to avoid rainwater problems. It is indicative of the felt’s deterioration. In the foreseeable future it will have to be renewed: an expensive job to do properly as it involves partial or complete removal of the roof covering.