For what major events does the merrymaking in Othello give opportunity?

The previous answer is right on the money, Iago takes
advantage of the merrymaking (and Cassio's weakness for drink) to set his plan in
motion.  Another interesting thing to note, however,  is how little of the actual "dirty
work" Iago does.  He only introduces the alcohol; it is Cassio (fully aware of his own
weakness) who drinks it.  And, when he recounts the events of the brawl to Othello, he
need only relay the facts of what has happened.  It is worth noting that Cassio is
actually responsible for his own downfall, ultimately.  So, the stripping of Cassio's
office is the main event that the merrymaking provides opportunity
for.

What is amazing, when considering the sequence of
events regarding Cassio that follow, is that Iago could only hope that it would go so
well.   Othello strips Cassio of his position, which leads to Cassio (at Iago's
instigation) asking Desdemona to help him resume his place, which leads to Desdemona
spending most of her onstage time pleading for the man that Othello grows to believe she
is having an affair with...and the events continue to build and add on from
there.

Well, it's quite a pivotal moment of merrymaking in
Act II, scene iii, and one that shows how able Iago is to seize the opportunity to set
his horrible plan in motion.