BabyRuth and others have already given you fine answers...what I will add is that poets and speakers, plus other literary authors often use it for emphasis through beauty. Here is an unforgettable line from Edgar Allan Poe which I still remember from childhood:
"the silken-sad uncertain rustling of each purple curtain..."
This to me is one of the most beautiful examples of alliteration, and it is from his poem THE RAVEN...read it all for yourself here:
It can be defined as the situation where there is occurrence of the same letter or sound. This can be at the beginning or they can also be connected words and it is generally the reputation of sound in a phrase. You can get more details on the definition at https://encrypted.google.com/search?q=What+is+alliteration.
One of the cleverest uses of alliteration I know comes from C.S.Lewis's "Narnian Suite". Here are the first four lines:
With plucking pizzicato and the prattle of the kettledrum We’re trotting into battle mid a clatter of accoutrement; Our beards are big as periwigs and trickle with opopanax, And trinketry and treasure twinkle out on every part of us