Old felt becomes brittle
Bituminous felt can split around nails and deteriorate at the eaves. Small tears seen from the loft may be historic rather than the present leak.
Inside patches are limited
A patch stuck to the underside cannot usually be lapped beneath the material above. Water running on top can pass behind it.
Repair from above where practical
When tiles or slates are lifted for the actual roof repair, damaged underlay can be dealt with properly over a sensible area.
At the eaves
Old felt often breaks down where it reaches the gutter. An eaves support tray can be useful in suitable roofs, but it needs to integrate with the underlay rather than simply sit behind the fascia.