It's not cable, it's steel pipe. The tower serves several purposes.
First, it either bends or straightens the pipe, depending on whether they are spooling up or spooling out. Here is a shot looking up into the bending machine.
Second, it acts as the tension machine or braking mechanism. All that pipe between the vessel and the seabed is HEAVY, without a tension machine there it would unspool uncontrollably fast, wrecking pretty much everything. Sorry, this one is a bit blurry but it is a shot looking down into the tension machine.
Third, they occasionally need to strap on buoys, strakes, and beacons or sometimes cut the pipe and weld on subsea installations such as PLEMs, PLETs, valves, manifolds etc. In order to do that, they need a work station. That is what the big box thing is, a big work station with decks that can open, tilt, slide up and down etc. First pic is a buoy being attached to the pipeline.
This set shows strakes being placed on the pipeline in the top deck of the work station, then the straps are tightened with pneumatic tools in the lower deck. Keep in mind the pipeline is constantly moving the entire time from top (spool side) to bottom (sea bed). At full blast we managed to keep up a 10-12 meter per minute pipe movement rate. If you fuck up and they have to stop spooling, everyone on the crew gives you a massive dose of shit talking.
The entire tower can slide from left to right (port to starboard) and can tilt from 90 degrees down to I think 30 degrees. That last number I am pulling out of my ass, I cannot remember the exact figure. The tilt allows the ship to roll up pre-fabricated stalks of pipe from shore based installations. Here's a shot of the same vessel in port spooling up several miles worth of pipe, see how the tower is tilted? Walls, they become floors!
How much pipe can it hold? Well, that depends on the size of the pipe really. This vessel can handle pipe up to 20" OD. I think. Again, I can't remember the exact figure. Here's an idea of how many stalks we fabricated. Keep in mind, each one is 3/4 of a mile long and I think we ran 55+ stalks.