Allah Almighty commanded believers to mention His Beautiful Names in prayer, because no one can praise Allah better than He praised Himself. The Noble Quran says:
“Allah has beautiful Names. So address Him, calling Him by these Names" (Sura 7, verse 180)

Bukhari cites a hadith that says that whoever learns the 99 names of Allah will enter Paradise. The number of names of Allah is not limited to 99, because the Almighty has innumerable perfect attributes and Beautiful names, the essence of which only He Himself knows.