The rune does 4% of your Maximum health as damage, and heals you for 2% of your maximum health, as well as increasing your maximum health each time it is used. This is coupled with taking the health stat stick and overgrowth. Because laning phase lasts longer and Overgrowth no longer requires you to actually kill the minion, this results in Caitlyn being at 1000+ health at 10 minutes, where other adcs are only at ~800. On top of this, by taking the resolve tree it allows you to take Demolish, which is very useful in collecting plates.
Grasp may not seem that strong, but given Caitlyn's nature and the nature of botlane, it is very useful on Caitlyn. Even though Grasp deals less damage than PtA and less healing than Fleet Footwork, nstead of taking a rune like PtA that only gives damage, LT that only gives attack speed, or FF that only heals, Grasp gives both damage and healing. Caitlyn is a very poke-heavy ADC, able to autoattack the enemy without getting in range of danger, meaning she gets free damage and can heal back damage from minions or the poke from a ranged support, as well as increasing her maximum health. This makes poking her practically impossible and makes it take longer for her to kill in the long run, coupled with Overgrowth. Furthermore, Second Wind further drops her ability to be poked by healing back %of her missing hp after being damaged by a champion.
Lastly, plate gold causes intense snowballing in the current meta. If you can get all five plates without letting your enemy get many, you will be very far ahead in gold and exp. The nature of this is that in order to get plates, you have to push. If the enemy is pushed under tower, they can't deal damage to your tower. Caitlyn is a lane bully with her range who can poke under tower, meaning a good Caitlyn can force the enemy to back, which means free plates. Demolish allows her to faster push the plate, giving her enough time to take the plate then recall and get back to lane before the enemy has even reached the tower.
As a rough summary, Caitlyn being a lane bully that can safely harass the enemy from a long range, the Resolve tree favours her in many ways. The health gained from Overgrowth and Grasp makes her heal more from Second Wind and Grasp, as well as deal more damage with Grasp. Caitlyn can shove the enemy in then harass them under tower, and any damage taken from a poke support will just be healed back by Second Wind and Grasp. When the enemy gets low health from the constant harass, Caitlyn can quickly take a plate and recall because of Demolish and make it back to lane before the enemy can push the tower. This can create a large gold and exp lead, and a quick snowball in the hands of a skilled player. The Resolve tree is very strong on her in the current meta.
#Where is the Data Coming From?
You're from EU, so you should be seeing it a lot. It's played a lot in high elo in EU and KR, but NA is slow to get into these trends and is strongly against trying new things even if they're better (which is why NA tends to lose global competitions). They'd quicker flame a new strategy because it isn't tradition than give it a try and adapt to the meta. My girlfriend and I get flamed because I queue Jungle and she queues Mid, then I pick Twitch and she picks Leona, and we funnel the gold/xp from both jungle camps and mid cs to me. It's a common strat in other regions, yet NA just refuses to adapt.
However, EU, KR, and CN are really quick on adapting and changing strategies. I'm surprised you haven't seen it yet. If you look at probuilds, almost every ADC in EU that picks Caitlyn is running grasp on her, and a couple of people in KR are doing it.
https://www.probuilds.net/champions/details/Caitlyn (specifically select EU and KR)
Riot does have internal API they use to detect trends in multiple settings. They've released information in the past that used statistics from their own internal API. Here's an example:
https://nexus.leagueoflegends.com/en-us/2018/03/dev-making-matchmaking-better/
#In other words...
yes, Grasp is good on Caitlyn and there is plenty of data and reasoning to support that claim.