what an odd OP.
What is the point of the word "tepid" - just say "slightly warm"
What is the point of the word "loquacious" - just say "talkative"
What is the point of the word "profound" - just say deep.
In fact feck it, what's the point of any word? Why not just grunt and point.
Ok, to answer your q seriously, because it means something totally different to "pour" and is a quick way to say (as Moles said) "pour into another, more practical or more decorative container".
If you say
"I poured the wine" it has a totally different meaning to "I decanted the wine".
You could say "I poured the wine into a decanter" - but that's rather long winded and still includes the word "decant" to boot.
You could also say "I poured the wine into another vessel in order to breath, and in order to leave any undrinkable residue in the original bottle" but that's still more long-winded.
Are there any other words you would like to vent about? . Come to that - why say "vent" why not "let off steam, either in a metaphorical or a literal fashion".