No sacrifice can be performed without the help of curds and clarified butter. The very character as sacrifice which sacrifices have, depends upon clarified butter. Hence clarified butter is considered as the very root of sacrifice.
ऋते दधि घृतेनेह न यज्ञः सम्प्रवर्तते। तेन यज्ञस्य यज्ञत्वमतो मूलं च कथ्यते॥