There are three common cooling system types that benefit from a Thermo-Bob:

Bikes with no Thermostatic Control
All popular motocross bikes from the major manufacturers come with no thermostatic control at all. Coolant is constantly pumped through the radiator whenever the engine is running, resulting in engine coolant temperatures that are too low until the outside temperatures are hot (where the radiator sizing was established in the first place). In cooler weather, the fuel injection system reads the cold coolant temps and overfuels, as it sees the bike still in ‘warm-up mode’. The end result is frequent oil changes as you'll dilute the lubricating oil with fuel that has washed down the cylinder walls, shortening engine life.  With a Thermo-Bob, coolant that is below the thermostat temperature bypasses the radiator(s) and goes back into the engine for another pass. This is exactly how automotive cooling systems have worked for decades.

Bikes with cold Thermostats
Some of the KTM models (XC-W or EXC for instance) have a factory plastic thermostat with bypass, which run too cold in the snow because the thermostat is rated at only 158°F, plus they have a large thermostat bleed hole still sending flow through the radiators all the time. This works reasonably well when used as a dirt bike, but on a snow bike it overcools the engine. The anodized aluminum Thermo-Bob replaces the factory plastic thermostat assembly, raises the operating temperature to 180°F in all 4-stroke kits, has a very small or no bleed hole in the thermostat at all, and a threaded port for a 1/8-28 BSPP temperature gauge.

Bikes with cold thermostats and no radiator bypass
Kawasaki’s thermostat bikes (such as the KLR and KLX) suffer from two problems: the factory thermostat is only 158°F AND they have no radiator bypass at all, which results in engine cooling relying on hot/cold/hot/cold ‘splashes’ of coolant, which promotes the cylinder going out of round and resulting in increased oil consumption. The Thermo-Bob solves both problems. If you’d like more detail, here is substantial data on this subject. (I have over 180,000 miles on my Thermo-Bob equipped KLR650 that has not been rebuilt, and it still only uses about 10 ounces of oil per 1,000 miles of use).

We have different Thermo-Bob families: the Thermo-Bob 1, 2, 3, and 4. One is not better than the other, as all function the same. The difference is simply the geometry to best fit your bike’s cooling system hose layout.