What is the use of phone keyboard dialing alphabet

I may be misunderstanding your question but if you mean the letters below the numbers on the phone keypad (e.g. the "ABC" below the 2), one use of them is for companies to advertise with easier to remember numbers. For example, Apple advertises the number (800) MY-APPLE which is dialed as (800) 692-7753 The words are easier to remember and the numbers on the keypad make it possible to translate the letters to which numbers you dial.