Companies buy other companies for 1 of the 2 reasons.
-A business that is fundamentally profitable to add to the portfolio
or
-Market Share. Which is basically 90% of all tech acquisitions. Build users, get sold, gets monetized.
What Discord has, which MS doesn't, is an all game and platform encompassing gaming social platform which allows MS to get a foothold on this market segment without needing to build their own tools and woo users, while simultaneously deal blows to its direct competitor like Epic, Steam, Nintendo and Sony as it now owns the platform many of their users use already; imagine getting GamePass ads on the official Playstation channel. Discord have interest in selling because there is no guarantee they would remain "gamer central"; their value is market cap, not their profit.