Not everyone is a "Mental Giant". That app for the bicycle speedometer is a life saver for accurately spraying high cost herbicides when your main means of rate control is your ground speed. Especially with my HST. Sure your speed may not matter much when your spraying your lawn or pasture with 2-4-D for dandelions. But there are times when it is important.
Years ago I was able to get a small "make up container" of an (at that time) somewhat exotic herbicide for killing grasses in corn. (Sweet corn is also a grass.) It was also one of the only ones available at the time for safe application in sweet corn. The tiny container had about 8 oz of dry granular herbicide and that package size was being discontinued because everything was tending towards much larger containers with the smallest being in the 2 to 5 lb. size or more.
That little 8 oz. box cost me just under $200 at the time. The application rate was something like .33 oz per acre in 3 to 10 gallons of water per application (two applications spaced 10 days apart.) The active ingredient by weight made gold look cheap. To make it last as long as possible we calculated the amount/rate for "band spraying" the rows. (3 acres of sweet corn) Herbicide was weighed on an electronic gun powder scale and the tractor ground speed was critical to get the right application rate for grass control and to not waste the product or damage the sweet corn with over application. It was amazing how well that product worked and how close we could finish with just oz's left or maybe 25 feet to manually hoe if we ran short.
Being able to use a speedometer app in single MPH with display to the nearest 1/10 MPH on a hydrostatic transmission sure makes the calculations and application easy. And for sure beats trying to watch the rows while counting and timing the tire rotations on the old geared tractor because the the RPM/Speed gauge wasn't accurate enough in the varying conditions.
I do agree that a person should be proficient enough to do the rate calculations manually and use the online calculators to verify your own math until you trust that calculator. But if you ever need to do some delicate band spraying in a crop susceptible to damage from over application, an accurate speedometer is a must have. My B2650 HST did not come with anything close to that.
There's most likely other users that also have a need for one for their particular applications, too. It's a big world out there. LOL!