A skip isn't just about access to new material it is also about getting into a class with kids who are themselves further along in their development. Real abstract example:
If your kid is on average speaking in 10 word sentences and kis their age are typically speaking in 5 word sentences and kids a year older are speaking in 7 word sentences, then the year up is at least more compatible on language use.

Things like abstract reasoning, language sophistication, etc. advance as kids get older.