Why did a dial phone have letters on?

for a long period (when I was a boy), phone numbers consisted of a Word plus a number (Pennsylvania 6-5000 was turned into a song)

so it would be easier to memorize.

Added: Thus Pennsylvania 6 would be PA6 or 726-5000. I am from Ohio and our number was BA8 XXXX or 228. People all around the country had similar numbers.