Let me get this out of the way first. "Composed Upon Westminster Bridge" is an odd topic for Wordsworth. Why? Because he is a romantic poet. A key to the era of romanticism is nature. Romantic authors heralded the beauty of nature, the perfection of nature, the spiritual power of nature, etc. Nature is awesome. So when Wordsworth writes this poem and talks about how beautiful a man-made city is, it's a definite departure from a standard romantic topic.
That brings me to the first language technique. Exaggeration. "Earth has not anything to show more fair:" Really? The city of London is the MOST beautiful thing you have EVER seen? He is exaggerating of course to help him sell the point that the city in the morning IS beautiful. The hyperbole continues in lines 9-11, too. He says that he has never felt so calm and has never seen a more perfect sunrise.
A simile is contained in lines 4-5. "This City now doth, like a garment, wear
The beauty of the morning."
That line is also an example of personification
.People wear clothes, not cities. Other examples are present as well. The sun is referred to as a male, and the last line indicates that the city has a heart.
As to why Wordsworth would choose hyperbole, similes, and personification, I think it has to do with the fact that it makes the poem more accessible to a casual reader. The language of the poem is not "high" language. It's simple and everyday. Something a common person would be educated enough to make sense of. People frequently use hyperbole. I hear it all the time from students of mine. "It was the best movie EVER!" And then I hear that same kid say the same thing the next day about a different movie. Wordsworth does the same thing.
The comparisons ground the poem, too. One of the best ways to explain a concept is to relate to something that the audience is already familiar with.In the poem's case, the human body. All in all, Wordsworth's language techniques make this one of his more accessible poems.