Shaykh Saalih al Fawzaan (hafithahullaah) mentioned: “The reason behind mentioning dhikr and invocation following ablution is to combine both physical and spiritual purity, for ablution is a means of physical purity, whereas dhikr and invocation (implying monotheism and repentance) are signs of spiritual purity. In this way, one becomes in the most perfect state of purity (combining both physical and spiritual purity) and becomes pure enough to be admitted to stand before Allaah in prayer, imploring Him humbly and submissively.
“Mulakhkhas al Fiqh”
Vol. #1, Chapter 7, Page 43 (Eng. Version)
Shaykh Saalih al Fawzaan