why did he do it?

[Legolas]

why did tolkien put so much poetry in his books?

[Ilúvatar]

Okay I have to say it - because he was a Poet

Alright maybe the questions should say "Why do you think Tolkien put so much poetry in his books?"

I think Tolkien really liked how poetry expressed what he wanted to say in such a beautiful way.

[Lomadia]

I agree with McDLT, and would like to add that poetry was the form in which many stories of a similar style (Beowulf, the Kalevala, et al.) were told. So much can be told about a culture by its poetry and song, and I find the songs to be some of the most enjoyable parts of The Lord of the Rings.
Elen sila lumenn omentielvo,
Lomadia

[Manwë Súlimo]

I have no idea why he worte so many poems but it was probably his way of making the characters personalities come out.

For a good example of Tolkien's poems, read the Lays of Beleriand (a little prehistoric, but truly a work of genius).

[Elanor]

I quite agree, for me poems were never a favorite thing, but after reading LoTR and The Sil, I feel completely different.