Do battery strips prevent an iPhone's power from cutting out? I have an iPhone SE. I wanted to get a new battery. I made the mistake of taking it to a non-Apple vendor. They broke my iPhone and I wonder what role the battery strips (missing from my iPhone) play. 

The phone starts and I can log in. Everything is fine until the phone suddenly cuts out and goes dead. This happens both when charging and not charging. It doesn't shut down, just cuts out.

The shop staff told me it's a software issue. I just don't believe them as I don't see any process happening when it cuts and the state of the phone when it's back up is different from when it gets cut out.

The shop staff also lied to me about what was and wasn't in my phone when they returned it. Apple told me the battery strips were missing and the device should be replaced (I took it to Apple after the other vendor broke the phone).

So now I'm wondering what the significance and purpose of battery strips is? And, whether or not getting new strips might fix my phone?

Edit: 
I forgot to mention that I requested my original battery be put back in. So this cutting out problem happened with the replacement battery as well as with the original battery after their tampering.
Battery strips are adhesive, stretchy "tape" strips. The purpose of them is to keep the battery in place, as well as slightly insulate the battery from the casing. The insulation means that sudden applications of heat or cold to the outside case (such as for example taking the phone out in intense sun light, or placing it on a very cold surface) doesn't immediately transfer the heat/cold to the battery via the aluminium casing.

If you're not exposing the phone to these changes in heat/cold when the cutout happens, it just sounds like you've received a non-Apple battery replacement of low quality.

Usually these cuts outs happen when the phone hardware uses more power than the battery can provide. As the power draw increases, the current (i.e. volts) are lowered slightly. With a bad battery, a normal power usage can lower the current so significantly that it is below the requirements of the CPU. This means that the system just shuts off immediately.

It is obviously not healthy for a phone.

I see it as very unlikely that new battery strips will solve your problem, unless you specifically see the problem when shaking the phone, or exposing it to heat/cold. Your problem could potentially be solved by replacing the battery - but then you'll also want battery strips installed ofcourse.