As an introverted person, I'd very much appreciate the option of subscribing "quietly". So if I check a box, twitch alerts for this particular sub (including chat announcement) don't trigger, but I get to continue supporting my favorite streamers. Users would still have the option of using the "regular" sub alert (probably by default), for example for x-yr anniversaries, congratulatory PB/WR subs, etc, etc. Basically, sometimes you feel like being social, and sometimes you don't. Having lots of fanfare for subbing is not something everyone wants.
Indeed subscribing when a streamer is not streaming is an option, but the idea is to lower the barrier for supporting streamers via subs, and I imagine there's a large percentage of twitch's viewership that is introverted and/or has social anxiety and would appreciate this feature. This doesn't discourage engagement because the target audience for this feature is probably already lurking! Streamers and twitch will benefit from the greater number of subscribers. Win-win all around.