Why is Agarose gel purification mandatory for direct sequencing of PCR products?

If gel purification is not performed and only a purification kit is used, it often leads to double peaks or messy peaks in the sequencing results. This is mainly caused by non-specific amplification products or incomplete removal of the original PCR primers. Most so-called PCR "purification kits" primarily recover the product but do not effectively purify it. They certainly cannot remove non-specific amplification products, and often they do not completely remove all PCR primers. These residual primers can participate in the sequencing reaction, causing messy peaks.