Thiruvaradhanam is a special austerity, unlike the regular rituals. While performing thiruvaradhanam we have to touch Emperuman’s deity; so, we should be clean and hygienic.
It becomes an offense if we touch Emperuman when we are not pure; so, we should wear clean clothes and urudhva pundram grants us purity. Hence, we should wear madi vastram and urudva pundram while doing thiruvaradhanam.