I agree that it was probably to keep her under control but I'm not sure about opening new rifts. Comstock and Fink (was it Fink? I forget) spent a fair amount of time researching it, hence the whole theory that Comstock became sterile by working so mucuh with rifts and why he then went and got Anna. The fact that they were able to open the rift to Anna in the first place shows they already had the technology to do it but perhaps they were experimenting on her to make it more stable?
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