* It will massively increase your marketability: C++ is one of the most popular languages around, and it will stay this way for at least 10-20 years. Most desktop programs are still written in it, most AAA games, and most operating systems, most multi-media data processing uses either C or C++, and it's still the most popular language in the embedded space. Wherever execution speed is a goal, C++ is the lingua franca. As an iOS dev, these might be markets you didn't consider before, but knowing C++ will go a long way towards unlocking them for you, should you ever want to go into them. It will increase the potential places of employment for you like very few other languages could.

* C++ is powerful. It lets you do stuff that's out of reach in a lot of other languages. Have you ever thought "gee, this inner loop costs too much runtime but is easily vectorizable. Man I wish I could hardcode this piece of code in SSE intrinsics (or other modern CPU instructions) or write inline assembly"? Or "man, I could actually already compute this at compile time if the processor would let me" or "If I could store this information in the upper bits of the memory addresses that go unused, I could squeeze the data in a cache line, making cache misses almost a non-issue"? C++ makes these things (and many others) available to you, if you ever want to.

* It will massively increase your software-craftmanship: There are very few popular languages that will expose you to such a breath and depth of programming language concepts, From bare-metal pointers and references, manual memory management & smart pointers to template metaprogramming. At least for me, no other programming language has given me this many instructional "aha" moments, because C++ makes a lot of stuff explicit that is just hidden away in most other languages. Even if you end up not using C++ much later on, knowing and understanding the concepts of C++ will make you a much better programmer in other languages as well. And will make learning other new programming languages much easier. Haskell and LISP are the only two languages I can think of that had similar effects for me.

Contra (Arguments against learning it):

* C++ is difficult. It has massive footguns and error messages (especially in templated code) are sometimes not very helpful. A lot of things that are easy in other languages are suddenly hard, and manual. Some bugs are absolutely non-trivial, hard to understand and to debug, on a level that you haven't seen before. With great power comes great responsibility.

* C++ is old: This may just be a riff on "it's difficult". But on several occasions, it'll get on your nerve. The build and link system are outdated. And C++ in 1998 looked different than C++ in 2017. And the language is still evolving.

You do not learn C or C++ if your goal is to become a regular Joe developer and you are not heavily focused on high performance and security. If you are more focused on security and performance there is a good chance you learned C and C++ so you could create modules for the web server, middle ware, operating system, etc. that you are using to improve performance and or security. You also need to know C and C++ to troubleshoot bugs or performance issues that may occur which will require you to go under the hood and review the source code for the language which is normally in C or C++, and if your using a debugger you will need to know some assembly from time to time.

It is always ill advised and frowned upon to become another copy paster and not understand what you are doing especially when using a programming language with full access to everything the operating system has to offer. It is like going to a job interviewing saying you know C++ and have never programmed any production quality code in C++ and the only thing you have done is some tutorials online. When some tough problems come up you will not be able to troubleshoot the problem first and be on hold waiting for someone to update your question online with something you hope is correct but you will unfortunately not be able to trust and verify the solution.

It is always the best course of action to take something you see as hard and take it as a challenge to learn it and become proficient so you are at least at an intermediate stage and are not struggling with the basics of the language. This will give you the ability to talk with others on a more professional level to solve any problems that will come up so the conversation will be constructive. It also feels good when you look back at something you thought was so hard and develop highly secure and high performance applications that you thought were not possible when you wrote your first few lines of code.

bonus : you don't have to learn a new language every decade.

Personally I want to focus on what code is doing, it's also important to understand how it's doing something, but typically I don't care that much about performance, I just want language that will produce valid code without too much effort and thousands little details that you need to be aware of and with languages like C++ you need to pay high attention to code you write and debugging can be horror story, once something goes bad.

For me, if I need to go low-level, Go is sufficient. It's readable, performant and it's harder to shot your feet with it.

I recommend you avoid trying to master C++: it's a gigantic waste of your time, of your brains and of your productivity. Lots of useless complexity. Just like learning Perl is (been there, done that, too).

OTOH, C is a language worth learning well.

In fact, I am sad to say that unless your job requires it, I can't really think of any reason to learn the language. It relies on a mixture of paradigms that are easier learned from other languages where they are isolated. And for lolevel stuff, you are better off learning C.