On the necessity of

Source: here.

On the necessity of mantrasiddhi and its primary means(puraścaraṇa in this context)

वीरचूडमण्याम् कालीतन्त्रे च:-

पुरश्चरण-हीनस्य मन्त्र-
सिद्धिर् न जायते।
आदौ पुरष्क्रियं कुर्यत्
नियमेन यथाविधि॥

वीरतन्त्रे-

आदौ मन्त्रस्य सिद्ध्यर्थं
पुरश्चरणमाचरेत्।
ततो सिद्धमनुर् मन्त्री
काम्यकर्माणि साधयेत्॥

उत्तरतन्त्रे-

सर्वस्वेनापि कर्तव्यम्
पुरश्चरणमुत्तमम्।
अन्यथा नाधिकारः स्यत्
तस्य पूजादिषु प्रिये॥

योगिनीहृदये -

गुरोराज्ञां समादाय
शुद्धान्तःकरणो नरः।
ततो पुरष्क्रियां कुर्यान्
मन्त्र-संसिद्धि-काम्यया॥
जीवहीनो यथा देहः
सर्वकर्मसु न क्षमः।
पुरश्चरण-हीनोऽपि
तथामन्त्रः प्रकीर्तितः॥

यामले-

अकृत्वा तु पुरश्चर्यां यो
यत्कर्म समाचरेत्।
तत् तस्य चाभिचाराय
जायते नात्र संशयः॥

रामार्चनछन्द्रिकायाम्-

संसारे दुःखभूयिष्ठे
य इच्छेत् सुखमात्मनः।
पञ्चाङ्गोपासनेनैव
रामं भजतु भक्तितः॥
पञ्चाङ्गोपासनं भक्त्या
पुरश्चरणमुच्यते।
एतद् +हि विदुषाम् श्रेष्ठं
संसारोच्छेदकारणम्॥
नानेन सदृषो धर्मो
नानेन सदृशन्तपः।
नानेन सदृशं किञ्चिद्
इष्टार्थस्य तपोधन॥

Some points to be noted from the above quotes

• It is essential for kāmya rites - Without puraścaraṇa,one does not do pūjā etc (basically mantrasiddhi at its various levels is essential for the deity to be responding to the ritualist: This is the point being hinted at here)
• The classic simile of a lifeless body being unable to perform any work; and a mantra that is not siddha is employed in the Yoginīhṛdaya
• A yāmalatantra even goes as far as to say that one who does rites without doing puraścaraṇa, his rites are akin to abhicāra
• Rāmārcana-candrikā,a text on Rāma worship notes that the puraścaraṇa ritual is also the cause of breakage of the bonds of saṃsāra; hence this is not merely recommended only for kāmya rites alone. And the ritual is also praised highly. This sentiment on the importance of mantrasiddhi at various levels is thus,not something that exists only in Śākta works but can be seen in Vaiṣnava works as well.