Scientific Meaning of Mantra  
In yagya we chant mantra. Mantra is a particular sound with a divine meaning and intention in Sanskrit, which produces a high-energy resonance. This resonance influences the person's physic and spirit and also the person himself for whom Samkalp (the oath) is taken. This resonance makes one feel better, energetic and confident to perform the best. In the other hand, it influences the opposite side of the person to be soft for him for an action. But the most important effect of mantra is that it absorbs the particularly needed sound energy of the planets and cosmos and passes in person's body for whom the Mantra is being chanted. 

Reach of the influence of a Mantra 
For a remedy purpose Yagya is most influential. There are three energy sources in this universe - Light, Sound & Magnet. Light is very good system but it has limitation; it can't reach beyond the wall or a non-transparent medium. Magnet is also very effective but it needs a metallic-medium to pass it's energy through. But the sound is everywhere reachable that's why it has been used most. As Mantra is based on sound system, it guarantees the best effect of Yagya. Moreover the Yagya is absolutely natural process and thus peaceful and long lasting too. The most important benefit of Yagya is that it is 100% positive. It means, it shall give result in any case, whether 99% or 90% but always positive. It never gives negative result. One more benefit of Yagya is, that, it has no reverse effect even you do wrong Yagya; because, in every Yagya, only Vedic Mantras are chanted and spiritual rituals are performed. Reversely, if you chant a Tantrik Mantra in wrong / incorrect method, you get reverse influence.  Therefore a Yagya can be called - FULL PROOF. Not only this, if you suppose not to see benefit of Yagya, still it's devotion and donation and has eternal influence for your soul and karma. This is the peculiar reason why we choose Yagya to promote as remedy because more or less, correct or incorrect, however you do, it has always result and positive result. This is the reason why Yagya was 10000 years ago too popular and now too is popular.