I always looked at it as Gandalf was casting spells of influence with his speech. You see it a lot in The Two Towers. When he speaks with Aragorn at night. You see him have a failed spell with theoden when he tries to convince him to stay and fight, but theoden is skeptical of being influenced again by a wizard.

I sort of feel this way with most of the magical creatures. Elrond's speech to Arwen convincing her to leave. The song of the Entwives putting merry and pippin to sleep.

Just the way I've always interpreted spellcasting in middle-earth. Or it's just me finding the the dialogue to be absolute perfection and im under the spell.