Bhagavad Gita

ತಸ್ಯ ಸಂಜನಯನ್ ಹರ್ಷಂ ಕುರುವೃದ್ಧಃ ಪಿತಾಮಹಃ ।
ಸಿಂಹನಾದಂ ವಿನದ್ಯೋಚ್ಚೈಃ ಶಂಖಂ ದಧ್ಮೌ ಪ್ರತಾಪವಾನ್॥೧೨॥
tasya saṃjanayan harṣaṃ kuruvṛddhaḥ pitāmahaḥ |
siṃhanādaṃ vinadyoccaiḥ śaṃkhaṃ dadhmau pratāpavān||12||

Sloka 1:12
Gist of the sloka:
To cheer up Duryodhana, the grandfather Bheeshmacharya roared like a lion and then blew his conch.
As a commander, it’s his responsibility to motivate his army and followers. Bheeshmacharya being the commander of the Kauravas first roared like a lion and then blew his conch, indicating his readiness for war, alerting and motivating his troops and also to indicate to Duryodhana he was ready for the fight. This was also a signal to the enemy camp about their readiness.
Conch sounds hold the power to ward off evil spirits up to the point the sound is heard. That entire area is free of evil spirits. A battle field contains such evil spirits and it is to drive away such spirits from the battlefield that conch is used.
Even in houses there is to be a practice of blowing the conch in all directions of the home to drive away evil spirits if any.