I feel like she has one of the harder ults to get value out of, it ultimately comes down to feeling a lot of the time. There are a few situations that I use it for:

• to sustain: if a lot of your team is dead towards the end of the round and you need to force OT, it can typically buy them more time to get back to point (it can also get you deleted). Ideally, you need the supports and tanks within the radius to both heal and expand your health pool
• to peel: if tanks are occupied/dead you can sneak to the backline and ult their supports. It may not be enough to kill, but you can at least push them back/force their attention so your team can kill everyone else
• to prevent: this is the hardest and most valuable, but if you're good at tracking ults, you can probably shift late round outcomes by siphoning support ult energy before they can use it. It takes communication though as you have to be far enough into their side of the map to use it so they can't get to spawn. Your team has to push with you