Keeping a beat is important, but so is playing the right notes and adding adequate expression. But keeping the beat is possibly the most important aspect, for a number of reasons.
The first reason why keeping the beat is important, is that it maintains the flow and integrity of the piece of music. This is extremely important when playing with other musicians or a singer. If you keep the beat and they keep the beat, then you are, at the very least, playing in time with each other. As you might already appreciate, when musicians play together, even if there is only 2 of them, if they get out of sync with each other, then the music sounds horrible pretty quickly.
Another reason why it matters so much to keep the beat in a piece of music, no matter what genre of music you are talking about, is that it helps keep the flow of the piece of music. Not everyone is going to agree with me on this one, but I believe it helps with preventing mistakes. Let me explain. If someone hits a wrong note and then stops or slows down, I believe it makes is harder to keep going and it can take a few or several notes until the piece of music sounds like it should. So this means that instead of being one mistake, there is actually a few or quite a few mistakes happening.
If you are keeping the beat and that is a really important focus for you, you are more likely to recover quickly from playing a wrong note, so the one wrong note is only one wrong note, not a few. As well as maintaining the beat throughout the piece, with the odd one note still in the correct timing, many people listening would not be able to detect the mistake at all. As you can imagine that from being a piano teacher for many years, I have attend or held many concerts for students. Trust me when I say that as long as the beat is maintained, ninety-nine percent of people would not hear the mistake.
Apart from these reasons, learning to keep the beat is a really important skill for any musician to learn, because it is such a huge part of the music itself. For some people it is a skill that comes very easy to them, whereas others find it very challenging. As someone who found it hard to master, I can understand how difficult it can be. Regardless, it is a skill worth persisting with and just because you might not find it easy, doesn’t mean you won’t be able to get good at it.
If you are finding keeping a beat a challenge, then maybe think about using a metronome. You could use a traditional one or you could get an app on your phone with a digital metronome. There is also now a type of metronome watch, where you feel the beat on your wrist, rather than hear it. I don’t know how good it is as I don’t know anyone who has one, but they sound good in theory. Counting aloud can also be useful however you have to be strict with yourself in trying to count the beat. Whatever the means,Keeping the beat is a very important part of playing the piano well.