Here’s a primer on CVTs in the American automobile market–not gonna talk about hydrostatic CVTs in agricultural or construction equipment.
The Toyota hybrid synergy drive is essentially an electric torque converter. Power from the combustion engine is sometimes sent to the generator then to a traction motor, which has a lot more torque than the combustion engine. In other situations, more of the power goes through the gears rather than the less efficient electric path. In a nutshell, power is converted to torque just like a hydraulic one. But this one has a battery that supplies power to the wheels. Imagine having a high pressure hydraulic accumulator that can shoot hydraulic fluid at the turbine without the engine running.
The frictional CVT are two large pulleys that clamps a metal belt or chain(more on those later). There are various web sites with diagrams explaining how ratio is changed. The drive capacity is dependent on the friction between the two metal parts(belt and pulleys), much like friction between train wheels and rail. This is why you don’t see OTR trucks hauling fruits to your local grocery stores using CVT. There’s not enough clamping force, or light enough material to withstand the clamping force without much deformation
CVTs use a special elixir that is liquid like at low pressure but turns into a solid substance when clamped between the belt and pulleys. In theory, there should be no contact between the belt, but we all know cars transported by trains are covered with rail dust. There are ASME papers that talks about slippage in a CVT. Just like engines, parts that slide past each other without lubrication wears out eventually. CVTs depends on parts not to slip and slip they do anyways
I know gears slip all the time as teeth mesh, but gears have lubrication that facilitates slippage. CVTs use traction fluid to resist slippage
The choice between belt or chain depends on the manufacturer. Chains can follow around the pulley in a tighter circle, which leads to wider ratio spread However, chains dive in and leave the pulleys at discrete points and this causes vibration.
Back to the OPs question. Because the cvt slips and wears all the time, the more you vary your speed the better since you are spreading the pulley wear across the entire range. If you use CVT in stop and go traffic or highway cruising, you’re subjecting the low or high range to constant wearing.