Discord’s main source of revenue is via the Nitro plan subscription, which offers various additional perks and access to various upcoming features if you have opted into the Discord Beta Program. Various users have been complaining that Discord has been advertising their Nitro plan whenever they launch/restart their Discord.
Sometimes these advertisements can be seen on other Discord servers as well. This is very much similar to what Microsoft has done in the past and faced various criticism.
Discord User Complaining