Hmm this is a rough explanations but because to be sure cells are made properly and not misfunctioning or going Rouge.
Cancer cells is an example of this, a cell goes rouge and starts doing it's function on it's own but outside of it's brother cells function slightly different turning into a growth or etc. The body thinks it is normal during the check body it's composed of the dna that passes this check.
viruses and other things don't pass this check and things like white blood cella come shut it down.
that's why doctors poison the area they find the cancer. to send white blood cells there to shut the area down not passing the check.
