I write only in English. Why would I use UTF8?

You should use unicode on your web site, because you do not know for how long you only will write in English. One day you may want to write the correct accents for Běijīng or you may make sure that your readers know that you are writing about the province Shǎnxī and not the neighbouring Shānxī. And what about writing Łódź like they do themselves in that city?

If you have built your web site around a non-unicode encoding, you cannot do that.

And if you are a mathematician you may one day want to type ε or ∀ or ∃ or ∈.

And even if you just type normal boring English text, you may come into situations where you want to use a curved apostrophe in "I’m" or real curved quotation marks like ‘Hello’ or “Hello”. And what about writing that someone is 5′3″ tall?

With the non-unicode "Western" encoding, you cannot do any of that. You can of course use the straight vertical characters ' and ". I do so myself all the time. However, it is strictly speaking not correct. The sad thing with the Western encoding is that you cannot write it correctly even if you want to.

With Unicode you can write not only that but much more.