What is the purpose of the iron structures seen on the sides of many WW2 flak towers?

These Flak-towers are imposing structures, but rare. The two pieces depicted are only to be found in Vienna. The towers in Berlin didn't have them, and those in Hamburg neither. Only the last two of these towers built, not "many", in Vienna/Augarten feature these. And on "many flak towers" in general: only in three cities they were build, in pairs of combat and control tower, with those pairs built ideally three times in a city, forming a triangle for an overlapping field of fire. But for example Hamburg only got two of those. Berlin had a full set of 3 combat towers, Vienna as well:

There were 8 flak tower complexes in the cities of Berlin, Hamburg, and Vienna.

— Wikipedia: Flak Towers

For those pictured from Vienna

we see that in the district Augarten the only two examples have these structures, made of concrete, but not steel, or "iron". They were codenamed "Peter" and completed in January 1945. The 'round one' (sixteen-sided) was the "combat tower" and held the bigger cannons, the rectangular one was a fire control unit.

The pair of flak towers in Vienna's Augarten (district: Leopoldstadt) bears the code name "Peter". The battle tower and fire control tower were built between July 1944 and January 1945 according to construction type III. With its 13 storeys, the Gefechtsturm is two metres higher than the 12-storey Leitturm in order to bring their platforms to the same level. These were the last flak towers of the Third Reich to be built, and accordingly the most advanced. […] Particularly striking are the reinforced concrete cantilever beams on the outside for easier maintenance or repair of the platforms.

— Wikipedia: Wiener Flaktürme

For the protruding concrete structures we find the following description, first for the cannon tower, then for the control unit:

On the circumferential platform around the 11th floor, concrete supports were attached below the projecting round stands for the light flak, which served the scaffolding during the construction of the tower. These concrete supports can only be found on this turret. […]

Around the 11th floor runs the lower platform for the light flak. Just like the Gefechtsturm in the Augarten, concrete supports are attached below the gun emplacements. These are missing from the control tower in Esterhazypark. Furthermore, these projecting gun emplacements are supported on the lower platform from below by concrete pillars. The measuring instruments were again located on the top platform.

— Bertram: "Flaktürme Wien", geheim-projekte.at, 1999. (translated from German)

More on the practicality side: the round protrusions for light flak cannons were called 'swallows nest'. Drawing from experience in Hamburg/Heilgengeistfeld ('Gefechtsturm IV') and how one of those nests broke off after an attack, the protrusions were added to facilitate quick repairs. (Cf. — Hans Sakkers: "Flaktürme. Berlin, Hamburg, Wien", Fortress Books: Nieuw-Weerdinge, 1998. or "Die Flaktürme", Airpower.at)

The above is somewhat contradicted —or expanded upon— by the following observation:

The concrete supports in the upper quarter became necessary after the order was givento raise the height of the tower by another ten metres, which would have caused the wooden scaffolding to collapse under its own weight. Apart from that, these concrete supports were quite helpful for the construction of the lobes directly above them.

— Michael Froedrowitz: "Flak Towers", Berlin Underworld Association, 2007. (Original: — Michael Foedrowitz: "The Flak Towers in Berlin, Hamburg and Vienna 1940–1950", Schiffer Publishing, 1998. Tidbit copied here.)

These practical considerations during construction and repair might be visualised as follows, for the Augarten control tower shortly before completion:

But apart from this 'immediate necessity', it might also have been part of the planning for an eventual post-war reality, since these massive structure were in no case easy to remove, or even possible to remove, without severely damaging the surroundings that is. So compare the plans from the very start to make the concrete monsters more palatable fo a civilian landscape after the war.

The war time facade plans for the Berlin Zoo bunker:

And what they fantasised about the desired giganto-Staufian looks from afar after the war:

(img src: Foedrowitz)

But this intention to use them for a future facade is unproven speculation, even if published in otherwise seen as 'reputable' magazines/newspapers. However the stones for these medieval mock-ups in front of the monstrosities were supposedly already quarried and partially ready for shipment from France, only for D-Day to become a slight inconvenience for that.