Mary will correct me if I'm wrong but the main reason low tow is taught
afaik is for cablerelease malfunctions.
If the gliderpilot is unable to release the tow cable he will signal this to the tug and assume low tow position. This will enable the tow pilot to take the tow combination back to the airfield and land in the safest way possible.
This enables the glider to land first, at which point the tow pilot can cut or release the cable and land ahead or go around.
In Australia we used to be taught this (landing on tow) during ab-initio but it was discontinued as the training risk was perceived to be more than the risk of it actually being needed - it was quite difficult to do! It's a lot easier for the tug to release from high tow ("abnormal tow" to us!) and land with the rope dangling.
I believe that the reason we use low-tow downunder (and almost nowhere else, surprisingly) is that we had a double (triple?) fatality after the glider got too high and neither tug nor glider could release .. the low tow position gives both parties more opportunity to recognise the glider getting out of station and release in time.
Interestingly enough, after 30 years of towing in both low & high tow I find it is generally easier to teach in low tow as getting out of station is visually much more obvious.
And to contradict another comment made, my experience here in Australia has been that it's better to go to high tow for a long retrieve as it doesn't require as much effort, and it's also more efficient.