Agreed, if it ain't broke don't fix it. That being said, it's good to know what everything on your boat does and how it does it. The lean/rich knob on a carb is used to adjust the fuel/air mix going into the combustion chambers. By twisting the needle in, the relative amount of fuel to air is lessened thereby making a "lean" mix (i.e. not very much fuel to air). This generally results in a hotter, faster burn which in turn generally raises the rpms slightly. Conversely, backing the needle out, increases the relative amount of fuel to air being combusted thereby resulting in a "rich" (a.k.a. "fat") mixture. With more fuel in the mix the combustion is somewhat slower and less violent thereby lowering the rpms slightly. It may seem a little backwards, but imagine a campfire. IF you want it to burn hotter, faster, better you blow on it to increase the air mix and speed up combustion, whereas dumping more gas on the flame may/may not increase the activity and may actually smother the fire if added too quickly. The trick is to dial in a steady idle at low rpms, which means you have to find a balance between the carb setting and the timing (rate of fire) of the motor. This is best achieved with the boat on the open water, allowing for 10-15 second intervals in between adjustments of the needle to allow for it to take effect. You generally only need to make small adjustments on the order of 1/8-1/4 turn. Here's another handy link for dialing in your carb should you need it.
Note: the terms "lean" and "rich" also apply to the fuel/oil premix you use on a 2-stroke motor. A "rich/fat" mix has a relativel high proportion of oil in the gas, whereas a "lean" mix would have relatively less oil in the fuel. It's always better to err on the side of caution when it comes to mixing your fuel. Better a bit too much oil than not enough. Too lean a mix will not provide enough lubrication for the internals of the motor and may lead to an overheat/seizure.
https://forums.iboats.com/showthread.php?t=167352