Why: there is no one who renders greater service to Krishna than this, nor shall there be anyone more dear to him on earth than this, ranking this act of sincere teaching remarkably highly among all possible forms of service or devotion.

In life: helping someone else genuinely understand something that matters, at the right time, with real care, is ranked here surprisingly highly, arguably higher than many other forms of service people more commonly consider more impressive or sacrificial.

Teaching: don't undervalue your own opportunities to genuinely help someone understand something important, at the right time, with real sincerity, this verse ranks that specific act remarkably highly among all possible forms of meaningful contribution.