As you may already know, there are A LOT of functions you can map to any of your wheel buttons. Whilst there are some that you may not need or use, depending on your preferences, this is a list of the ones I recommend
Pit Limiter: Unless you set the Pit Limiter to automatic (See Post Assists & Realism), you need to have this button mapped. I recommend using it mainly because when you get used to it and have some experience, you can enter the pits faster without speeding after the pit entry line.
Starter: This is something similar to the pit limiter. You could set the starter to automatic, but this will almost always be slower than the manual procedure and would make you lose some times in the pitstops
Ignition: Again, a setting to have in manual mode to gain time in the pitstops. Turning it off stops the engine immediately and you are faster this way than in automatic mode. Remember to turn it on once the Pitstop countdown starts and hold the starter to turn the engine on when the countdown reaches 0 (it actually works when 1s left, but do it at your own risk)
Flasher: Light signals are the only way drivers have to communicate with each other (remember not using the chat during qualifying and race!). You can use it to signalize the driver in front that you’re going to pass them, helping to make the overtake safer. Remember that continuos flashing is not seen as good sportsmanship and it may be penalized in some communities like ThePitCrew (see https://thepitcrew.co.uk/blog/rules/)
Blinkers: Similar to the flasher, the blinkers are another way to communicate with the other drivers. You can use one side to indicate you’re going to stay to this side, for example to signalize a lapping car that you are going to facilitate the pass. It is important to remark that you signalize the side you are going to stay, not the side you want the overtaking car to pass you. You can also indicate that you’re going to enter the pits, as in some tracks an unexpected pit entry could cause an incident. When you activate both blinkers, it the same as activating the hazard lights, which you can do some situations to signalize danger (for example if your car is especially slow due to damage).
Another common use of hazard lights is to “apologize” for some situation to the car in front or behind, as well as to “thank” another driver after letting you pass.
Cycle view: This function basically changes the car view between Cockpit, chase cam and hood cam. Even if I always stay in the cockpit cam without displayed steering wheel, I do use the hood cam for the Pit lane and for very extreme wet conditions, where the water spray barely let’s you see anything (I know, it feels like cheating)
Cycle MFD Up/Down: Use these to cycle between pages of your MFD. You don’t need to map both functions though, one is enough for most drivers.
Increase/Decrease ABS: This functions is useful to change the setting during the race, especially with changing conditions (higher for a wet track)
Increase/Decrease Brake Bias: I personally use this a lot. Depending on how the car feels, you can change it while you’re driving, shifting the braking pressure to the front or rear. When the track gets less grippier, you should decrease it to avoid front tyres lock ups.
Increase/Decrease Engine map: This function is useful when you have to save fuel and want to use a map with lower consumption, or even if you want a less aggressive map for wet conditions. Mind that each car has different engine maps, so check this out before you decide which one to use: https://www.assettocorsa.net/forum/index.php?threads/ecu-maps-implementation.54472/
Increase/Decrease TC: Basically the same as the ABS functions, but for the traction control
Increase/Decrease TC2: Some cars have also a TC2 that you can adjust while driving. Remember that if you have some electronics settings not mapped, you can still change them via the Electronis page in the MFD.
Shift button: This is actually one of the best functions. You can choose any button to work as Shift, and then map 2 different functions to the same other button, but one of them only working while your shift button is pressed too. That way you can have more functions that buttons mapped to your wheel.
Next/Previous Dash Page: It’s a non necessary binding, but useful on some cars to toggle between tyre and brake temperatures.