Why does the dialect matter? Can't we all just use "standard English"?
Well, first of all, there is no one official dialect of English that's used everywhere in the world. There isn't even a universal way of pronouncing England that's spoken consistently throughout the British Isles. In fact, there never has been a single absolute standard for how to spell or pronounce words in the English language. Instead, there have always been strong regional distinctions in pronunciation, dating back to the ancient language we now call "Old English", which actually sounds closer to modern German than to modern English.
Couldn't we just take some symbol, (say ê) to represent whatever sound I make when I pronounce the vowel in "met", regardless of how some other speaker pronounces that sound?
It turns that that we can't always do that, because different dialects of English don't just have different pronunciations; they also have different numbers of vowel and consonant sounds. In other words, two different words may sound the same in one dialect, but sound different in another dialect. So, you would have to spell those words the same way in the dialect where they sound the same, but in two different ways where they sound like two different words. That means you have to pick a dialect.
But, is this really a big issue? That is, are there actually any commonly used words where this problem really occurs?
Some English dialects pronounce the words "cot" and "caught", or "Don" and "Dawn" exactly the same; others don't.
Most British people distinguish the vowel sounds in "Mary", "merry", and "marry"; most Americans don't.
British people often pronounce the sounds "w" and "wh" distinctly, so "which" and "witch" are sound like two different words. Few Americans do this.
Many British people omit the "r" sound at the end of a after a vowel if it's the last sound in the word. Most Americans don't.
Outside of Scotland, "loch" and "lock" usually rhyme. Within Scotland, "loch" is pronounced with the same "ch" sound found in the German words "ich", or "Bach".
So, if we want to represent all sorts of common word and sound patterns, we have to pick a dialect which describes how to do it.
Why can't we just choose the most "universal" dialect with all the sounds of English in it?
There isn't one. Every dialect includes some pronunciations that are not found in other dialects, but excludes other pronunciations, so there is no such existing dialect to go by. You could try to invent one, but that would be a lot like inventing a new language.
But, isn't there a way to write all the sounds from all the dialects of English?
There are many ways, most of which overlap and/or contradict the others. This is part of the reason why English spelling is so confusing. There are many more sounds in English than there are letters in the English alphabet, and some of the spelling combinations reflect old pronunciations that we don't use anymore in modern english, like the "kn" sound in "knife" and "knight".
Can't you just invent new symbols for the sounds that don't have symbols?
Yes. In fact, this has already been done for all the sounds that exist in any human language on Earth. These symbols are called the IPA (the International Phonetic Alphabet), and professional linguists use it to describe and compare languages. Most people don't use it, though, because it's got a lot of sounds that they don't know how to pronounce, or even to recognize.
Why is dialect such a big deal? Can't all English speakers just switch dialects without a problem?
Learning sounds from other dialects often isn't easy. Native speakers of English grow up learning which distinctions in sounds to listen for, and which differences to in sounds to ignore. So, learning to hear and pronounce distinctions in sounds that you've trained yourself to ignore all your life is something that's very hard for most people to do. Many people find that the hardest part of learning a new language is pronouncing sounds that don't existing in their native tongue.
Without practice, English speakers sometimes can't even hear the difference between two words that are distinct in other dialects, such as "caught" and "cot", where the difference is in how rounded the lips are when the vowel sound for the word is spoken. For example, I can't hear this difference even when I try, because it's not used in the part of Canada where I grew up (southwestern Ontario).
The bottom line: In other words, in order to properly describe how to pronounce English, you really do have to pick a dialect first.