Tiles protect against normal rain, but they are not always 100% watertight against strong wind, accumulated snow or low pitches, and that is where the under-tile waterproofing membrane comes in.
When it is mandatory or highly recommended
It is practically mandatory on roofs with a reduced pitch (below the minimum recommended for the tile type), in areas with frequent snow, or on roofs very exposed to wind where water can get in under the pieces. It is also always recommended in refurbishment, as an additional safety layer against tiles that may shift over time.
Breathable membrane or vapour barrier
The breathable membrane lets out moisture that may be generated from inside the home while repelling rainwater from outside, the most suitable option on lived-in roofs or ones with thermal insulation. On cold roofs with no use of the interior space, a standard waterproof membrane without breathable capacity may be enough.
Why it isn't a superfluous expense
Omitting this layer to save on the quote passes the risk of leaks on to the client, which may not show up until the first heavy storm, when it is already far more expensive to locate and repair the source under the laid tiles.
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