Not just the 'what' or even the 'why', but the 'how'.
I suppose the first thing to admit to is that I am not a 'purist' photographer. For me, the camera is just the beginning, and the computer and software allows for total image control. So much so, I will often go out with the express purpose of shooting separate elements to be combined later. Before you pick up the first stone, remember, many of the early black and white masters happily spent hours in a darkroom burning in a better sky from a previous plate, in order to achieve a more stunning result.
The trouble now with digital photography, is that it has become like owning a computer, there is always a newer toy out there just after you have entered the game, and you can get left behind unless you keep throwing money at it.
I'm probably in the process of being left behind, but fighting hard to keep up. My basic camera is the Canon EOS5D. To go with this my first lens was the Canon 24-70mm 1: 2.8 L USM zoom. The decision to go down this route was based on two misconceptions on my part. The first was a thought along these lines... "I spend a lot of time in Egypt, in deserts, therefore it would be good not to keep on changing lenses, thus letting all the sand and dust in. The zoom lens can stay on all the time, and I will have no dust issues." Sadly, it doesn't work like that.
A zoom lens is like a lung, as it is turned one way it sucks air and dust in. When it is turned the other way, it blows the same air out, but manages somehow to leave all of the dust on the mirror and sensor! The result, a massive retouching exercise, especially on those deep blue skies.
The second misconception, was that the lens would be nothing short of brilliant. Sadly too that doesn't work. Firstly, f2.8 is pointless as it doesn't even approach sharp under f11. Secondly, it's soft round the edges, no matter what f stop. A total disappointment. Unlike the, crisp, Canon 100mm macro.
So when the time came to add a wide angle to the kit, I did not look straight away in Canon's direction. Several professional photographer friends talked of Sigma, with a lot of good adjectives, like, 'stunning', 'crisp', and for my benefit, 'reasonably priced!'
I have to first confess fighting the idea of an alien brand being attached to the camera, then I remembered my expensive piece of sub-standard, disappointing, Canon 24-70, and soon relented. The result, the Sigma, yes Sigma, 20mm 1:1.8 EX DG. I have never regretted this decision. I didn't realise a lens could be sharp all over without leaving the lens cap on, and sticking a pin hole in it! I love this lens. Wide without massive distortion, and it lets me get sharp shots in narrow tomb passages, even with very low levels of available light.
The Sphinx shot below was taken with it. 
LENS USED SIGMA 20mm 1:1.8 EX DG
So, when it came to add a telephoto to the kit, I went straight to Sigma, without even a side glance at Canon. I went for the 70-200 1: 2.8 APO DG HSM with Optical Stabilizer, plus the APO Tele Converter 1.4 x EX DG. This is nothing short of 'rediscovering photography', and I haven't put it through its paces yet. Not even put it on a tripod, just hand held. This is a lens that can be sharp at f 2.8. The picture below is a first effort, and demonstration of the crispness. Those whiskers can cut!!! 
LENS USED SIGMA 70-200 1:2.8 APO DG HSM Hand held
I will soon be taking it off to Egypt and am really looking forward to it, even though when on the camera, it's about the maximum weight limit I want to stagger round with. It's good to suffer for one's art!
As a total Sigma convert I would like to point you in their direction. Sigma Imaging (UK) Ltd