With incandescents, HIDs or LEDs I can understand their purpose - to prevent glaring. Or - on the contrary - a Fresnel lens may help to equalize the illumination level of an area.
However, fluorescent don't glare and can't be focused effectively. So why most of fluorescent fittings have dents of various shapes or they're frosted?
It's unnecessary, IMHO and it may reduce about 20%-30% of light output (my guess). What do you think about it?

The cover (Diffuser), is to protect the lamp from weather conditions (Water, rain, humidity, etc...).
Most IP54 or IP65 fluorescent fixtures, have a diffuser to seal the lamp.

I can understand that. But why the unnecessary optical properties? (dents, frosting, ...) A clear cover from glass/plastic would protect from those weather conditions as well and it'd block less light (I think that only about 5% of light would be wasted this way).
The only reason I can think of is the aesthetic one - to prevent people from looking at "ugly" tubes. Or, the clear cover could make the fixture look "cheaper".

Btw. LG member monkeyface posted a picture of a fluorescent fixture with a clear cover and it doesn't look bad at all.

This is a road lanterns, and road lanterns needs covers to protect the lamps.
The americans and the british, were the pioneers in use the protection covers in their fixtures as diffusers. The main purpose of the cover, is to protect the lamp against temperature differencies and weather conditions. The diffusing is to make the lamp invisible to the user, so the entrie fixture profile will be the lightsource itself, as in some cases, the consumer wants to see a fixture and not lamps.
My generic Hyundai 21W T5, had a plastic white diffused, which made the lamp invisible, so I removed it so I can see the lamp itself (Which on the way, reducing the heating of the electronic ballast).

I think it has to do with making the fixture look like a single source of light instead of individual tubes. I'm fine with clear prismatic diffusers or the open-grid type, but I don't like the solid white ones.

I think it has to do with making the fixture look like a single source of light instead of individual tubes. I'm fine with clear prismatic diffusers or the open-grid type, but I don't like the solid white ones.

It could be so in some cases - but see this one - it's a single tube fixture and yet there are dents blocking the light in the downward direction. This is what I can't understand.

If the lamp is in direct view, even the fluorescent is too bright (do not confuse brightness with light output), so the light needs to be diffused over way larger area.

On your previous link there is an attempt to even out the illumination underneath the fixture: It is supposed to be installed on a ~3m high ceiling of some larger area warehouse, parking lot or so. That is in some aspect similar to the street lighting: You want to cover the whole area with as uniform light as possible with as few fixtures as possible.
Naked tube radiates the light the same way in all directions. As the floor underneath is the closes, it will get most of the light, even when it's area is rather small, while the large area further away get covered by less light.
So this fixture is designed to take the light from underneath the fixture and throw it to the side, so to cover the more distant part of the floor.
Of course, it is by far not as efficient as with the HID's on streets, but as you have to illuminate the whole area and not just a stripe (the road,...), still it helps a lot.

I don't know what you want to use it for, but this optics would be suitable for e.g. garage ceiling, but not as much for the function of a shop light...

That sounds logical, thanks. I also came to this conclusion after I had posted onto this thread the last time.
I've found that this kind of diffuser is usually called prismatic.

@If the lamp is in direct view, even the fluorescent is too bright...
I don't even find T5 tubes too bright but I might be more resistant than other people (especially those wearing glasses). I spent a few months in an open-space office with PL downlighters. They were perfectly fine for me but many people found them unbearably glaring.

The downlighters have louvers to limit the angle to about 45deg from vertical, so light really only down. As normally you do not look upwards, that should be sufficient measure to prevent glare.
They are usually used with small spacings just to get enough light for the office work environment, so the need of many fixtures to cover the floor with rather solid illumination with such small angle is not a problem.

For the "garage fixture" the application is different: Cover rather large indoor areas with not as high lighting levels (warehouses, garages,...; just to see on your steps), so they are likely spaced more away from each other, so redirecting the light to the ~70deg off vertical is beneficial to reasonably cover the widest area with minimum fixtures.