Pinyin Writer helps you type pinyin with tones which you can then copy and paste into other apps.

Why write "Ni(3) hao(3)! Ni(3) jiao(4) shen(2) me ming(2) zi(4)?" when you can write "Nǐ hǎo! Nǐ jiào shén me míng zì?"

Most pinyin keyboards convert pinyin input into Chinese characters but Pinyin Writer leaves you with your original pinyin with tone marks that you can copy into other apps.