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
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)
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
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.
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.