Why is it necessary to purify PCR products?

It is necessary to purify PCR products in order to remove any unwanted substances or leftover reaction components from the PCR reaction, and to ensure that a clean, reliable DNA sample is used for downstream applications.  

PCR is carried out to produce multiple copies of a specific DNA segment. At the end of the process, primers, buffer components, nucleotides and enzymes may still be present in the final DNA sample. The presence of these contaminants in the sample decreases read length and signal intensity, often resulting in unreadable sequence data. 

Purifying PCR products removes these unwanted substances and ensures that only the specific DNA is isolated, making it suitable for sequencing and other downstream applications.