I'm sure it's useful for training and mentorship, but that's frankly not your employer's problem these days. People switch jobs every year, and if you train up staff only their next company is really going to reap the benefits. If learning on the job raises retention then it's only marginal. If anything it makes people "worth more" and they more easily find the next gig that comes with a pay bump
You can easily skill up by:
- watching conference videos on YouTube
- during code review
- go on your lang subreddit or forums and read about how other people skin their cats
What you want is basically unstructured interruption based code review ~ which is nice for the junior but horrible for productivity over all