The reason is that as you grow and expand as a photographer you never know the things you are usually trying to accomplish. You can't always predict what you will be doing as an artist a few years from now, or maybe even tomorrow. What if you wanted some perspective controls to tweak an image. You may have never done it before and may only do it once but for that image it may be important for you. Having a program like Photoshop means there is a far better chance that whatever you are trying to accomplish there is probably a tool set in Photoshop that will accomplish it. That doesn't mean there aren't other platforms that can accomplish the same task, just that Photoshop can do almost anything in terms of digital manipulation, even if there are individual programs that handle specific tasks better. Think of it as Photoshop can accomplish almost any task well, instead of one or two thing incredibly well and then not anything else.
For what it's worth it's not the only program I use, I find Helicon Focus is better for focus stacking and HDR Effects Pro better for the HDR that I do. Photoshop can assemble panoramas but not nearly at the level PT GUI can, and that's why I use it. But if I didn't have enough of a need for all those other programs and only thought I would use them occasionally then you could use Photoshop to do any of them. Just not to the level that those programs could accomplish those individual tasks.
When I am done editing in HDR Effects Pro, Helicon Focus and PT Gui I still go back to Photoshop because of all of the editing tools it has and I have been using it for decades so I am familiar with the platform. But there is no denying that Photoshop is the Swiss Army knife of photo editing. If you have it you are more capable in a digital darkroom environment.
Just my two cents. The bottom line is any photographer should use whatever works best for them. But I have known photographers that buy two or three programs before buying a Photoshop Cc subscription.
-Joshua
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