Many muslim converts accept Islam because the belief that God is One and He Alone should be worshipped without any partners makes sense to them. In other words, their intellect accepts this message as the truth. When it comes to what God commands us to do, then we have to rely on revealed knowledge, because no matter how hard we use our brains we still wouldn't know the commands/laws of God unless He reveals it to us.
This is where the preservation of scripture is important, because we want to live by the guidelines prescribed to us by God. Muslims recognise that there are mainly two sources of knowledge, one being from our intellect/empirical senses, the other being revelation.
One can come to the conclusion that Islam is the truth based on their intellect alone, but then in order to be muslim (one who submits to God) they would then need to rely on revelation. Using intellect alone you won't know how many times to pray a day, how to pray, how to fast, what actions are forbidden, etc.
Between revelation and intellect, revelation has more certainty. Our theories/empirical studies can be wrong.. aka Pluto can be a planet one moment, and the next it's not a planet. Revelation on the other hand can never be wrong. The catholic church mixed the two, which is why they had a big problem when Galileo Galilei went against the theory that the earth was the centre of the universe, because it was a theory that they strongly supported (because the Son of God was on earth so it makes sense the earth is the centre of the universe).