Short answer: Yes—doing Atthai (eight-day fast) without fully knowing “why” still gives punya. But the quality and strength of that punya depend on your inner feelings (bhāva).

Simple explanation:

• In Jain Dharma, punya (merit) and pāp (demerit) are bound mainly by our intention (bhāva) along with the actual act (dravya).
• If someone observes Atthai with basic faith, non‑violence, humility, and without pride or show, punya arises even if they don’t know all the details.
• However, when Atthai is done with the right understanding, right faith (samyak-darśan), and pure feelings—free from anger, pride, deceit, greed—the punya is deeper and more elevating.
• If it is done for name-fame, competition, or with anger/ego, the result decreases, and it can even lead to bondage of pāp.

About Atthai:

• Atthai is an intense tapas of eight days of fasting/observances. Both Śvetāmbara and Digambara traditions honor it. All agree that bhāva‑tapas (inner purity) is higher than only dravya‑tapas (the external fast).

Practical way to make your Atthai truly fruitful:

• Keep maitri (friendliness), kṣamā (forgiveness), and vivek (discretion).
• Do svādhyāy, pratikraman, sāmāyik, and japa as you can.
• Remain humble; avoid publicity and comparison.
• Offer the fruits for right knowledge and progress on the path, not for worldly gain.

Remember: Even punya is still bondage; the highest aim is to purify bhāva, stop new bondage (saṃvara), shed old karmas (nirjarā), and move towards moksha.

Sorry, I can only answer questions related to Jainism.