When the YouTube algorithm chooses which content to promote on the featured page it looks at watch time more than any other statistic. A creator can help increase the watch time on their videos by creating content that their target audience will enjoy. The same creator can do this by having different avenues to display their content.
This will establish a niche for the audience and they will come to expect content that is not main channel material and be more inclined to watch. This implies that a YouTuber might benefit by having many YouTube channels to help categorize their content. Doing so will narrow down the audience and increase watch time.
People who subscribe to secondary channels are looking for that niche content. Compare this to posting niche content on the main channel where certain content is expected. It might not pull the same viewership as a video posted on a secondary channel where niche content is expected. The main channel viewer might not care for content that a second channel viewer might.