they are generally to be used in conjunction with under tile felt/to repair damaged end felt, but can still be employed if updating the facia and soffits, but unless your tiles are in good order (no missing, cracked or other wise damaged ,) it could give you problems...

staining down the facia, usually occurs when rain is either driven back from the tile ends, by the wind, or if the over hang to the tiles over the guttering is not adequate ...

hope this helps