Purification of PCR products is generally not necessary. You can use unpurified PCR products directly, as long as the total volume of PCR products in the Assembly reaction is 20% or less. If greater amounts of PCR products are used, a column cleanup kit is sufficient. It is advantageous to gel-purify the target DNA fragment if the PCR product is contaminated by either non-specific amplification products, primer-dimers or large quantities of unused PCR primers.