What సూర్య remedies are for
Surya remedies are for authority, vitality and the relationship with the father — the parts of life the Sun signifies. They are prescribed when the Sun is weak or afflicted in a chart, not as a general tonic; a strong Sun needs no strengthening.
When it is under strain
The corpus describes an ill-placed Sun as low self-esteem, absent energy and confidence, and friction with authority and with the father. That is the pattern these practices address.