When a Brahmin boy is 8 years old, he has to go through the Upanayana Samskara and after that he has to wear the Janeva (sacred thread in Marathi) and he is informed to offer Sandhya every day to Sun.
Why Brahmins are supposed to offer Sandhya everyday?
Having rinsed, the mouth, as laid down in the Mantram, one should again rinse it. Then sprinkling one's own self with water, as described in the Mantram, one should throw palmfuls of water. (12)
By virtue of the boon conferred by Brahma, whose birth is not known, the Rakshasas Mandeha fight every morning with the Sun. (13)
The handfuls of water, thrown by the Brahmanas and inspired with the mystic verse Gayatri, destroy all the Rakshasas passing under the appellation of Mandeha (14)
Harita Smriti, Chapter 6.
The highlighted portion above gives the "reasons" why a Brahmin should offer water each day to Sun at three Sandhya times (sunrise, noon and sunset).
BTW, Sandhya is a must for all Dvijas i.e compulsory even for any Kshatriya and Vaishya who have gone through the sacred thread ceremony.
And, if a Brahmin fails to do Sandhya then that is an offence and he has atone it by doing penances.
If a householder, out of mistake, does not perform his Sandhya-adoration or deviates from the performance of a Snataka-vow, he should fast far the day. (66)
In consequence of willfully [neglecting the Sandhya], the foremost of the twice-born should perform the distressing penance [of Prajapatya for full one year, and should perform the Chandrayana, [if he does so] for livelihood. [And] by making a gift of kine, [he is] purged of [his sins]. (67)
Usana Smriti, Chapter 9.
He, who out of carelessness fails to perform the Sandhya once, either in the morning or in the evening, should, with a concentrated mind, recite, after bathing, the Gayatri for a thousand times. (63)
Atri Smriti
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This is spoken of as the threefold Sandhya wherein exists the dignity of a Brahmana. He is not called a Brahmana who has no reverence for it. (15)
As serpents cannot approach Garuda, so imperfection cannot approach him who fears the non-performance of the Sandhya and who is always given to bathing.
Katyayana Smriti, Chapter 11.
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Unless badly jeopardised in health, or otherwise incapable, a twice-born one should never neglect the timely performance of his Homa and Sandhya rites.
Vyasa Smriti's last chapter.
So, a Dvija should always perform the Sandhya rites without fail.
