I have them in my tank.
I researched heating cables about 30 years ago and thought that it would be a good idea.
The whole create convection currents to keep the substrate healthy, and supposedly bring fresh nutrient rich water to the roots made sense.
I bought Dupla substrate heating cables about 20 years ago, they run on 24 volts.
I have seen heating cables that don't use a step down transformer, they run on plain 115 volts. Sounds like an accident (electrocution) waiting to happen.
Finally got them in the tank about 13 years ago.
I had a little DIY transformer project, due to the fact the Dupla transformer is wired for use in Germany.
Not that big of a project, rather heavy 24 volt 12 amp isolated transformer, case, fuse, indicator light, and banana jacks to plug in the cables.
And you will need a normal heater also, I plug everything into a temperature controller.
The substrate is a bit on the deep side to keep roots from wrapping around the cables, and pulling the cables up when moving a plant. But looking under the
tank you can see roots all over.
And quite honestly, I really can't tell you if they doo anything!
They were more popular a few decades ago, but not so much now,,,,,, I think.
I believe the thoughts on substrate heating cables are, save your money, and put it towards getting a better something else, filter, light, whatever.