Why did she have to die over and over in this time loop?

Yeah I thought that was strange as well. Couldn't she just stay alive and test the machine? I guess I missed something. Also, in the alternate dimension, her mother was alive and well, but yet she still grows up to be the same crappy careless person that she was at the beginning of the first movie. I thought that her mother being alive would have changed her habits at least, but she was the same.

I think the idea was to reset before the killing spree at the hospital begins.

The only way to save those people was to fix the time anomaly before this happening. Or to stop the killer. She tried once, but that didn't help... so she instead focused on getting the science right - which incidentally meant she had to die before the end of each day.

Also evident in the end, when she ”buys time” while the scientist fiddle, and she goes to try and prevent the killings. N'est pas?

In the end after she helped prevent the hospital killings, the only reason she went back was to be with that guy again, only his relationship with Danielle never felt all that strong, and Tree and him were in the middle of kissing before the loop was closed anyway. It became a very weak reason for closing the loop and going back as she basically could have had everything she wanted without the loop closing - her mum back, her boyfriend, and Lori as a good friend this time.

She is destined to die bcos she is bound to see her friend getting stabbed and killer shud kill her this time to not have her as proof of his first murder.

So the movie could happen.