If you have ever heard the term holy trinity of lenses, which is a way of saying you need 3 lenses to cover as much range as possible - a wide angle, a standard zoom, and medium telephoto lens, the 70-200mm focal range is definitely one of them. Even in sports photography, while you might drop of one those in order to have a 400/500/600mm prime, the 70-200 will still be in that line-up 99% of time.
A 70-200mm lens should be in every (serious) photographer's kit. There are different versions (and brands) of this lens, but I'm going to cover Canon's EF 70-200mm f/2.8L IS II USM, as that's the one I own, and highly recommend. Most of the advice here applies to all 70-200 lenses though. Here are a few reasons why you should seriously consider getting it:
It's simply a workhorse of a lens
I've used my lens in all sorts of weather conditions, and it keeps working perfectly. Granted, when it's raining heavily - and that happens a lot Ireland - I use a rain sleeve to add additional protection, but that's me being extra protective. This lens performs well regardless of the weather, and it's built like a tank, it's meant to last, and it really does.
As said above, there are different versions of this lens from Canon alone, as well as from third party manufacturers. The weather sealing, build quality, weight, focus speed, and other features will vary, but they should all be great lenses. I'll go over alternatives later.
It's possibly the most versatile lens you can have in your kit
You can use it for indoor sports (the f2.8 aperture helps a lot in poorly lit conditions), outdoor sports, portraits, landscapes, concerts, weddings, it's all around lens. The focal range is very useful, and the constant aperture means you don't have to keep worrying about settings if you zoom in or out, you can focus on the action. Having a wide aperture also means you'll have great bokeh, blur out the background and get those distractions out of the picture.
Keep in mind if you're using this on a crop sensor camera (like the 70D, 7D, etc) you'll have to apply the crop factor of 1.6 for Canon, which means it becomes a 112-320mm. That can be a bad thing, or a good thing, depending on what you're using it for.
The quality of images this lens produces is amazing, tack sharp, and the color reproduction is great. The IS versions (image stabilization) also helps you get a smoother performance when using longer focal lengths, especially if handheld. It weighs 1.5Kg, but it's very easy to handhold for the duration of a sports event. If you get tired, you can always attach a monopod to the tripod collar the lens comes with.
It's an investment
If you're serious about photography, you want something that will last, and not have to be replaced in a few years. Camera bodies are replaced at a far faster pace than lenses, and they devalue a lot faster too. The mark II version of this lens came out in 2010, and 9 years later still is one of the best lenses money can buy. The Mark III came out in 2018 and for someone who has the previous version, the upgrade is not worth it in my opinion. Granted, the lens is not cheap, but once I bit the bullet and coughed up the money for it, I never looked back, and definitely do not regret it. Quality and performance does come with a price - here I find it's justified.
For the past years, both Sigma and Tamron have upped their game considerably, which is great for everyone - it forces Canon (and Nikon) to push for innovation and all customers benefit from it. The price of their lens is usually much cheaper than the Canon/Nikon counterparts, and in some cases, the quality exceeds them.
If you want to stick to Canon, they also have F4 versions, which are also cheaper, lighter, and the quality is pretty much the same. If the one stop of light difference doesn't bother you, it's a great alternative.
Is it the right lens for you?
If you're just starting out and want something other than your kit lens, and you're on a budget, the answer is probably no. The first lens other than the kit lens I'd recommend everyone get is the Canon EF 50mm f/1.8 STM. That's the lens that got me properly hooked into photography - it's the rabbit’s hole ;) If you've passed that phase and you're looking into some more serious, then the 70-200 is what I would recommend, without thinking twice.
If you're thinking about purchasing the Canon's EF 70-200mm f/2.8L IS II USM, a different lens, or anything else, consider using the Amazon links provided below, it costs you nothing extra, and I get a tiny kick back for referring you to Amazon, which helps this channel / page.
Canon EF 70-200mm f2.8 L IS III USM: https://amzn.to/2OV5KdH
Canon EF 70-200 mm f2.8 L IS II USM (the one I use): https://amzn.to/2Ic8zFE
Canon EF 70-200mm f4 L IS II USM: https://amzn.to/2Id1aGi
Canon EF 70-200 mm f4 L IS USM: https://amzn.to/2OOyc0t
Tamron A025E 70-200 mm G2 VC USD Lens for Canon: https://amzn.to/2OKS30y
Tamron 70-200 F/2.8 VC USD Canon: https://amzn.to/2OLvSrj
Sigma 70-200mm F2.8 DG OS HSM | S Canon Fit : https://amzn.to/2K4P4l5
Sigma 70-200mm f/2.8 APO EX DG HSM | Canon Fit : https://amzn.to/2IcqX12
Also mentioned above:
Canon EF 50mm f/1.8 STM: https://amzn.to/2K8nJ1h