Knowing letters (A, B, C...) is just the first step. But knowing only letter names is not enough for reading and writing.
Letters tell us:
The names of the symbols (A = "ay", B = "bee")
But not how to read or pronounce words
Phonics tells us:
The sounds letters make (A = /a/ as in "apple")
How to blend these sounds to read words
How to spell words by listening to their sounds
Real-Life Example:
A child knows these letter names: C - A - T.
But without phonics, they might read it as:
"See-Ay-Tee" β not "cat"
With phonics, they learn:
C says /k/
A says /a/
T says /t/
οΈ They blend: /k/ + /a/ + /t/ = cat 
π§ Why Phonics Is Needed:
Without Phonics With Phonics
Can say letter names Can read real words
Can sing ABC song Can read a book
Struggles with new words Can sound out new words
Canβt spell correctly Can break words into sounds
Phonics is needed because:
Letters tell what a symbol is.
Phonics tells what that symbol does β how it sounds, how it forms words.
So, phonics turns letters into meaningful reading.
Without phonics, letters are just symbols.
With phonics, they become language.