Why if anyone wants to become a Muslim, is he asked to say the testimony of faith, “There is no god but God?”

The Phrase of Unity, “There is no god but God,” means there is none deserving of worship but God and all that is worshipped other than Him is false. This phrase denies everything the right to be worshipped, then it exempts God alone so that He is all that is worshipped for He alone is the Creator and it is He alone is the doer of acts, glory be to Him.
Atheists deny everything the right to be worshipped, they have not known God so they say, “There is no god.” Polytheists give the right of worship to God as well as other beings alongside Him saying, “There is no god but God, and the idols,” or “the stars,” or “the Messiah” , or “Aziz,” or “the Buddha,” and other beings that polytheists hold to be divine or divine like.
The Muslim monotheists are upon guidance, they deny all created beings the right to be worshipped and they believe that everything that is other than God is created, God alone is the Creator, none other than He deserves to be worshipped; He is the One worshipped who is deserving of all inner and outer forms of worship and supplication.
The reality of this phrase is that you should only trust in God, you should only rely on God, you should only seek assistance from God, you should only intend God, you should only desire God, and you should only see God. May God bless us with the realities of unity (tawhid), and God is Most High and knows best.