

Yes. It loads faster, it has integrated quality levels that increase while loading (so a web hoster doesn’t need to have 5 different copies of the same image at different qualities), it has better compression and it also supports more features. It can also be lossless. Most importantly, jpeg can be converted to jpeg xl losslessly, and it will have the benefits of jpeg xl.

The number is the signal you send to the program. There’s a lot of signals you can send (not just 15 and 9).
The difference between them is that 15 (called SIGTERM) tells the program to terminate by itself (so it can store its cached data, create a save without losing data or corrupting, drop all its open connections gracefully, etc). 9 (called SYGKILL) will forcefully kill a program, without waiting for it to properly close.
You normally should send signal 15 to a program, to tell it to stop. If the program is frozen and it’s not responding or stopping, you then send signal 9 and forcefully kill it. No signal is “better” than the other, they just have different usecases.