A lot of benifits have been said by performing Sandhya Vandhanam.Sandhya has two meanings--one the twilight-another sri.Gayatri's synonym is Sandhya.-Gayatri-Sandhya-Vidhya-Saraswathi---sandhya vandhanam gives-"Ojhosi-Balamasi-Braajosi--WHEN "Sukhla Dhaathu"=Semen is judiciously spent and preserved,it gives a benifit called "Urdhva Rrethas"--the enegy thus preserved imparts "Ojhas"--Dr.C.Dwarakanath says in his "Introduction to Kayachikitsa"--Ojhas is immunity.we may say something akin to that -In the works of sarva sri.Charaka, Susrutha,Vagbhata,Maadhava,Sarangadhara,Bhela (Samhithas) Ojhas is said to be of Two types--Apara(in the Heart 8 bindus and in the whole body Ardhanjali-I do not want to go for elaboration--This Ojhas gives Balam and Bhraajas(the Sanskrit Dhathu(root) "Bbhra" means scintillating light--what is called "Tejhas"--so sandhya Vandhanam is said to give immunity,Balam,Bbhrajakam(Tejas)=Aura--one can gon on write a big essay--our forefathers have carefully desighned an excellent "Nitya Karma" for us--One can do "Naimitthika Karma", at will- but Nitya Karma ,one can not do at will--One has to eat food to sustain life.
My colleague and best friend-a very orthodox Varanasi Brahmin Dr.Ayodhya Pandey(Sukhla Yajur Vedhi-Maadhyaanthika Zhaakha) used to say without doing sandhya vandhanam even one performs "Vaaja Peyam" it is something equivalent to feeding 1000 brahmins without giving one grain of rice to Biological mother--because vedham says about Sandhya-"Maathaha Itham Brahma Jushasva Naha"