The Qur'an was revealed in Arabic wording. Altering its consonants or vowel marks may change the word, and sometimes its meaning. Preserving the letters and vocalization is therefore central to accurate recitation.
First risk: substituting one letter for another.
Second risk: reading a short vowel or sukun incorrectly.
Third risk: neglecting a required letter quality so that pronunciation is impaired.
Tajweed provides a way to recognize and correct these errors. The reciter attends not only to written letters but also to the actual sounds they produce.