the benefits of extra gears are obvious to people familiar with automotive technology. Torque multiplication at low speeds can help with towing, pulling up hills, and just accelerating a lot faster. Using taller gears at highway speeds can increase efficiency and even allow cars to have similar performance with smaller motors that consume less power.
EV Truck seem to have enough torque but having a gear for highway would increase range. I don’t know significantly.
The Tesla Semi has three motors. Diesel trucks have just one engine. Once the Tesla Semi is up to speed only one motor is engaged. The three motor architecture does the “gear shifting” job.
Electric cars also have a type of transmission, but they’ll usually only have one gear, meaning no gear shifts are needed. The key difference between ICE and electric cars is that the electric motor in an EV provides instant power and torque to go the top speed without gear shifts. An ICE car needs to shift gears to reach certain speeds.
The Captain
Don’t know. But I find it interesting that early Formula E race cars had 5-speed sequential shift transmissions, but today’s cars are almost all single-speed. This in a race car that can hit nearly 200mph, but must still be fast enough at slower speeds due to the nature of road course racing.
Prior to your post, Captain, I was thinking "different technology → different solution, i.e., that with electric motors and their 100% torque from start, the ideal solution was probably something other than a transmission … and then you gave us one!
Tesla did! 
The Captain
Not just the semi! My little Tesla Model 3 has a front motor and a rear motor, and at highway speeds with less power demand, it turns off one of the motors for better efficiency. My overall efficiency over close to 6000 miles is 236 Wh/mi.