With no book of Esther, there would be no record left of what they did to the Persians. When people forget, history repeats. Without the OT we wouldn't be able to compare the story of the exodus to archeology and find that they were the slavers rather than the enslaved, and people would be more likely to fall for their claimed victim tricks. Without the OT, there would be no record of them ripping these doctrines off of the aryan religions. The OT is a testament against the excesses of priests, kings and yids, written by yid priests. Without the OT we would have no record of why they were destroyed three times, so that we can avoid such a fate. Beyond that there is a lot of information in there relevant to the nations.