Music, and sponsorship opportunities (for those with good subscriber numbers (~8k+).
The music and sound effects offered through EpidemicSound, AudioMicro and Position Music are going to be your greatest tools, let me explain.
Imagine you have some in-game music playing, but you edit your videos and have to play a jump-cut. Well, with these music outlets, you can disable the music in-game and add the music in post-production, improving the quality of your videos as there is no obvious audio stutter.