Is system useless without monitoring?

@kflanc You can use the system without monitoring, however, as you stated, it's effectiveness of course would be limited. I recommend you call SS and ask questions about how it would be used and then make an informed decision. To assist, here is a link to the type of plans, including self monitoring:

https://support.simplisafe.com/articles/alarm-events-monitoring/what-are-the-service-plan-options/6344794a013ba90af0bce6a4

One last thing. I would suggest your daughter contact her insurance company and see what type of discount she would earn with a monitored system.  Mine is quite large, due, per my agent, that I have five water sensors, besides CO, smoke and temp sensors.

Please post your decision here and experience if you get a chance. Best of luck.

To SS:  I had to try four searches before finding the plan page.  Indexing needs to be improved to get this information easier to get in front of potential and existing customers. And, again, stop the ADT like marketing crap and get rid of the X per day pricing. 31.99 a month for top tier Fast Protect Plan. See, not hard.

For example, would it be easy for someone to break in and just smash the siren?

This is actually two questions, isn't it?

First, "Would it be easy for someone to break in?" is a question we can't answer. How physically secure is their house? At a bare minimum I'd make sure there are security strike plates on every exterior door. (And I'd make sure you get the ones that are several feet long and and anchor them with 3.5" high grade screws at a minimum.) This won't completely stop someone from kicking in a door but it'll make it take much more effort to do so and they may bail before they can finish the job.

Second, "Would it be easy for someone to just smash the siren?" is a little easier to answer - it wouldn't be that difficult to smash the base station. That said, it should be noted that an intruder would need to find the base station to smash it. Also when the alarm goes off it will notify your daughter via the mobile app that their alarm has gone off. (Assuming they configured it to do so and I can't imagine why they wouldn't.) At that point she or her family would need to call the police or FD instead of SimpliSafe doing so.

So no, the system isn't useless without monitoring. It still notifies you when the alarm goes off but you need to call the appropriate emergency services. Also, you need to set the alarm. I know that sounds obvious but they're not going to get in the habit of setting the alarm then the system becomes far less useful, IMO.