One thing imo that a lot of isekais lack is the proper use of multiple characters being transferred into another world. Similar to Shizue, Yuuki creates a connection to Rimuru from his former world. Is it not needed? Maybe not, but it makes the conversation that Rimuru has with Shizue and Yuuki more heartfelt because it has the feel of seeing an old friend from your hometown. As Rimuru creates a place for monsters to be at peace with each other, this aspect gives Rimuru motivation to get along with humans. Despite being a slime, his personality is essentially human.
Also, this episode supplements the stuff regarding Shizue where she was forcefully summoned into this world and was used as a weapon. The summoning ritual Veldora mentions at the beginning of the series can also summon children. In which the power that we saw Shizue in habit, kids can't contain that in a healthy manner for more than 5 years.
The series did a great job in demonstrating how cruel it was for Shizue, but imagine these group of kids who got forced to live in this world and will be forced to die for things absolutely out of their control. By understanding Shizue's situation you can really grasp how important helping those kids was to her. You really understand what Rimuru calls carrying on her will.
Overall this shows the cruel part of this world and the human side of it can be sickening. These people are greedy, and it further illustrates that these individuals being summoned are looked as no more than tools. But it's even more gruesome to see that they don't care about the consequences for the people being summoned. Oh, it's a child? Guess we had bad luck better trying again.