This is the first "dedicated external signal" of activity on the most viewed new music on YouTube, according to the company. That's a big shift from before, where YouTube was previously focused on total views. You can still view all-time demand for artists and videos (now with some unspecified improvements), but it's clear that YouTube wants your eyes on what's happening right this second.
As to why? The trending charts are coming just days after Billboard started giving more weight to paid streams, but YouTube's Stephen Bryan was adamant in a conversation with Rolling Stone that this wasn't a reaction to playing second fiddle. It was just "unfortunate" that Billboard believed people with credit cards mattered more, he said, with YouTube representing a "more accurate" view of music. The company's Candice Morrissey told Music Ally in an interview that this was "very complementary" to the official charts in numerous countries, showing a more comprehensive view of what people enjoy.
However, YouTube may also be priming the pump. YouTube is expected to launch a new music service later in 2018, and it would reportedly meld both on-demand music with YouTube-sourced videos. The charts you see now wouldn't just represent free views -- they'd give some indication of what's popular on Remix. Music labels and media might give more credence to YouTube's charts if they knew the rankings were partly influenced by subscribers.