WUDU is a pre-condition for Salah. It shows Islam’s stress on purifying ourselves, physically and spiritually.
Prophet Muhammad (peace be upon him) said about the virtue of Wudhu: “When a slave makes ablution and rinses his mouth, his wrong deeds fall from it. As he rinses his nose, his wrong deeds fall from it. When he washes his face, his wrong deeds fall from it until they fall beneath his eyelashes. When he washes his hands, his wrong deeds fall from them until they fall from beneath his fingernails. When he wipes his head, his wrong deeds fall from it until they fall from his ears. When he washes his feet, his wrong deeds fall from them until they fall from beneath his toenails. Then his walking to the mosque and his prayer fetch him an extra reward.” (Malik, An-Nasa’i, Ibn Majah and Al-Hakim)
After we complete our Wudhu, one of the Du’as our Prophet (peace be upon him) taught us to say is: “Ash-hadhu An Laa ilaaha illAllahu wahdahu Laa Shareeka Lahu wa Ash-hadu Anna Muhammadan Abduhu wa Rasuluhu” (I bear witness that none has the right to be worshipped except Allah, alone, without any partner, and I bear witness that Muhammad is His slave and Messenger).
Sheikh Salih Al-Fowzan explained: “The reason behind reciting Dhikr and invocation following ablution is to combine both physical and spiritual purity.
Ablution is a means of attaining physical purity, whereas Dhikr and invocation (implying declaring monotheism and repentance for sins) are signs of spiritual purity.
In this way, one gets into the perfect state of purity (combining both physical and spiritual purity) and becomes pure enough to be admitted to stand before Allah in prayer, imploring Him humbly and submissively. (Mulakhkhas Al-Fiqh, Eng. version, vol. 1, ch. 7, p. 43)