Is it useful to buy Reddit subscribers?
For Reddit internal traffic → not much use.
For Google SERP ranking → has a potential indirect effect.
Google crawler looks at subreddits
Although the number of subscribers is not the only determining factor, under the premise of no negative signals, subreddits with high subscription numbers give Google the signal of "potential, popular, and community attributes".

The key is naturalness.
If it increases from 16 to 1000 at once, and there are no posts, no comments, and no upvotes, Google crawlers and Reddit anti-cheating are likely to trigger spam signals, which will drop rankings instead.