those marks on the speedometer are the max speeds for each corresponding gear.

when upshifting, you have a tach that you can use, to ensure that you do not exceed the redline. When downshifting however, it is more difficult to predict what engine speed will result if you downshift to a lower gear, at a given vehicle speed. More importantly, you cannot quickly tell if redline will be exceeded when the downshift is performed.

those speedo marks therefore are for downshifting. In my 5spd 190e, the "III" marking for 3rd gear is at (about) 100mph. If I am at 4th gear at 110mph and braking for a turn (or an off-ramp), and am planning to downshift to 3rd, I simply look at the speedo needle - when it gets below 100mph, it is safe to shift to 3rd gear, at which point it will be at redline in that gear. This is especially important if you are performing a heel-and-toe downshift (braking while downshifting).

there is usually no mark for the top gear because there is no other gear to downshift from.

But even in my 5spd 190e, there is no mark for 4th gear. This is because the car is not redline-limited in 4th, i.e., I can attain top speed in 4th with the engine below redline. Rather, it is drag-limited or governor-limited (or both). This means that i can safely downshift from 5th to 4th at any speed.

however, if I can take my car down a steep enough slope with the wind at my back and make it exceed the rated top speed while in 5th (but still below redline), then I cannot quickly tell what speed above the rated top speed will it be safe to downshift to 4th, when required, since there is no mark for 4th on the speedo.

same is true if you are coasting down that steep incline at a speed higher than the rated top speed with the transmission in neutral . You cannot quickly tell if it will be safe to place the transmission in top gear at that speed because there is no marking for it in the speedometer.