this is very late but i'll give some extra info (as someone who uses Sayu a lot) rainslasher is often used on Sayu because of the Elemental Mastery substat, so along side the extra damage against enemies with Hydro or Electro from rainslasher's ability (what Rathurue stated), Sayu also heals more because of that Elemental Mastery due to her 1st passive healing the party for 300 + 1.2*Sayu's Elemental Mastery stat
I wonder if this counts as aged like milk considering she rebranded her LL youtube account as Nimi Nightmare as soon as the kirin graduation and her youtube bio links to Nimi's twitter account now.
I'm not sure if she will go back to streaming because her inability to sit for an extended time due to injury meant she had to prioritize either her art or streaming instead of continuing to do both, and a lot of people would be sad if she dropped the art. Haven't been following that situation so not sure if she's improved though.