read somewhere its prob like 10-15 watts at most for HU. but the difference is huge between HU built in amp and aftermarket 2ch or 4ch amps. just look at the size difference. Aftermarket amps are huge and heavy.
The immediate difference is the sound. from HU alone, yes u can get nice sound, but your component speakers are made to receive at least 50 watts. so using HU alone is not doing it any justice.
How i know? i changed my HU and components last year. sounded much better than stock. this year, i added in amplifier and soundproof. Midbass from components sounded much stronger, higher frequencies sounded much clearer from tweeter, no more ear piercing sound.
In short, amplifier gives ur speaker the ideal power to perform, and sends a strong, clear signal from HU to speakers, thus producing the best sound a particular speaker can produce.