Alistar is a strong support, but for all his CC, he comes with a few weaknesses. He has to go all in when attacking the enemy; Headbutt (range of 650) puts him right onto the enemy champion’s range and Pulverize has a relatively small area of effect. Since Alistar’s only damage is melee, you can punish him when he approaches the AD or support by using ranged attacks/abilities. You can use these traits against him by warding the side bushes of the lane to stop him from getting into range for his Headbutt/Pulverize combo and by forcing him to use his Triumphant Roar by poking his health down. Poking is something Sona excels at, as Hymn of Valor (her Q, range of 700) plus Power Chord (her passive) allows for a surprising amount of damage at relatively low risk. If the bushes are warded, then you know where Alistar is and can keep him away from your AD carry with Q and passive poke.
Since their AD carry was Caitlyn, they have the advantage early game. Winning lanes is Caitlyn’s main strength, while Tristana is a complete monster late game (though she does have a really mean level 2 if she takes her Rocket Jump). Ideally, Tristana wants to get to mid/late game where her strengths shine. Playing near your tower and having thorough ward coverage is the ideal way of getting to late game without feeding the Caitlyn.
With Nocturne being their jungle and Alistar their support, the chance of a tower dive is pretty high. Alistar’s ultimate (Unbreakable Will. 50% damage reduction at rank 1) allows him to tank the tower damage really easily, and Nocturne’s ultimate (Paranoia) allows him to jump right to Sona or Tristana. You can have a better chance of seeing Nocturne’s ultimate coming by warding the tri-brush and near Dragon. Those wards cover the areas where his range covers bottom lane. The best way to avoid tower dives is to avoid Caitlyn’s Piltover Peacemakers (Q), and use Sona’s ultimate (Crescendo) defensively under the tower. By using Sona’s ultimate under the tower, you can quickly turn a gank into killing multiple enemies with the help of the tower.
Tristana and Sona win their lane by playing defensive and only engaging when they have the clear advantage from poking the enemy down. Even if they don’t win lane, if they can avoid feeding kills to the Caitlyn and stay relatively close on creeps slain (CS), then they come into mid/late game stronger.
She is speaking directly to her summoner's mind. As she says when you select her:
"Only you can hear me Summoner, what masterpiece shall we play today?"
From the League of Legends wikia:
Because Sona cannot speak, these (her in game) responses are understood to be magical telepathy with her summoner. This is seen in the fact that her external interactions (taunt, joke) are musical cues rather than words, meaning that, as stated, only her summoner can hear her words.
Source: Link
Best Answer
Sona Taric is an incredibly strong for a couple of reasons:
The use of both support items lets the botlane be even or ahead in gold while playing defensive. If Sona Taric has lane control they'll be very far ahead as Taric can get all the CS, share some with Targons and Sona will be collecting as much or more from Kleptomancy and the Frostfang line.
More importantly, late game both champions have incredible sustain. Two Taric heals and a Sona heal will bring someone from nearly dead to full. The only option is to all in the team in an attempt to get kill but Taric ultimate makes that impossible.
The speed from Sona's E and Taric support items and his proxy stun mean any team comp can just ran at the enemy in a death ball, be immune from Taric's heals and ultimate and Sona's shield and faster than the enemy. If the enemy team tries to split push they can pick someone off even under tower. If the enemy team tries to 5v5 Sona+Taric they'll be at a disadvantage as Crescendo and Cosmic Radiance are some of the strongest 5v5 ultimates in the game.
As you asked specifically for pro play, in pro play this combo is almost always paired with a Kindred, to ensure that you CAN'T kill them with the initial burst, and the heals will be very effective after her ult (which gives enough time to channel Cosmic Radiance). One of the teams in the MSI play ins ran this team with Kayle top, abusing the invulnerability mechanics to the max.
Some teams tried to play with Morgana mid to make someone trully invulnerable to both CC and Damage, but for the most part that seemed to lack enough damage to actually win teamfights in the late game.
In short, Sona Taric provides a lot of healing and durability to the entire team with Area of Effect shields, heals and invulnerability, as well as a speed buff. It also encourages death ball strategies that maximize the effect of Sona's abilities, which are quite effective with a short cooldown.