why we need to purify plasmid dna?

Answer 1

You dont. Although it makes things a lot easier if you remove as much stuff as possible and then put the plasmid into the appropriate buffer for your purpose.

Answer 2

Hi. From the web : Transient Tranfections
• In vivo and non-human primate experiments
• Toxicology, formulation, and biodistribution studies
• Plasmid based kits and diagnostic reagents
All require very pure starting material.