My understanding of BCCA supplementation is that there aren’t many scientifically valid uses for them but one that may exist is potential muscle sparing capabilities when training fasted.
While there is little evidence for using them for muscle synthesis there is some evidence that leucine can help recovery from training sessions that are completed fasted with the BCCA’s taken prior to the workout which don’t raise insulin levels enough to break the fast.
From what I read though even this is subject to much debate so you pays your money and takes your choice!