When a company or service is DDOSed the services are disrupted and sometimes taken down completely.
When a user or streamer is DDOSed the same goal remains. To disrupt or take them down completely.
Streamers are particularly targeted for the trolling aspect. When they are DDOSed they might be forced offline. This takes down their stream. And it hurts their ability to entice donations and subscriptions.
For example PSI Syndicate in the UK must go through a process to change his IP through his ISP. He eventually could no longer stream in the UK until he visited the USA as the rediscovery of his IPs and DDOS attacks became a regular issue. He has made mention in several vlogs how this has affected his regular revenue stream.
Sometimes they might be targeted for retaliation and to hurt their reputation. For example when Gaming For Good started a rival service for Twitch Alerts then TA was accused of view botting GFG's streams. This made GFG appear shady since they didn't have legitimate viewers. Eventually TA supposedly straight up started DDOSing GFG's streams all together. This however resulted in TA suing GFG claiming this to be unproven. If it happened or not is not my point. The point is how such methods could be used against another viewer for one viewers gain. And even though they are companies they were Twitch also streamers.
DDOSing streamers is now so common that Skype reconfigured it's default client settings due to gamers getting their IP addresses leaked for the purpose of DDOSing.
As far as regular users go DDOSing them might be for one of several reasons. Trolling, intimidation, retaliation over a personal matter, or even holding their connection ransom for digital items or even actual currency.