Reading in an undertone is helpful when you consider Bible books that are unfamiliar to you. To illustrate, imagine three scenarios.
مِنَ ٱلْمُفِيدِ أَنْ ‹ تَقْرَأَ هَمْسًا › ،‏ أَيْ بِسُرْعَةٍ تُتِيحُ لَكَ ٱلتَّأَمُّلَ ،‏ حِينَ تُطَالِعُ أَسْفَارًا تَعْتَبِرُهَا صَعْبَةَ ٱلْفَهْمِ .‏