Why do Hindus call re-conversion back to Hinduism as " Shuddhi " ?

Some act which other religions do are strongly prohibited in Hinduism ,Though they are allowed to other religions.For example cow slaughter is the religious ceremony in muslims during eid but cow slaughter is considered as a great sin in Hinduism. before marriage , eating meat or beef and many other things.So by doing religious ceremony they try to purify that person's mind body and soul from his past deeds which Hindus consider as impure before he comes to Hinduism.