It's to accommodate for the fact that we don't have the ability to select our targets with the same precision as PC players. They mentioned this was the reason why they changed Annie's Q and MF's Q. It adds skill expression as well while also opening up certain mechanics.
This is a big part of it, but it still depends on the ability. By default your abilities will prioritize champions and both Annie and MF Q are frequently used for last hitting and it would be painful to need to target these at minions that frequently. Skill shots also have some nice gameplay to them that we like, and allow us to put more power into these abilities. Tap targeting does a lot to lower the skill floor of our controls, though there are plenty of times where you want to aim, and we feel that accounts for the added skill that these being targeted adds.
In Kassadin's case we knew his laning phase couldn't be as punishing as it is in LoLPC and thus he wouldn't be using this as as much of a farming tool. It being a skill shot also allowed us to make it a silence due to it inherently having some unreliability.
We always want to evaluate what is best for Wild Rift when making a champion and that might result in changes to abilities.