OhTeeDee, You’re welcome! Yes, it is not mentioned in Tanakh, or even the Gemara, though I’m sure someone has tried to interpret pesukim as referring to gilgulim in some way. Ramban mentioned gilgulim, but the main way it was introduced into mainstream Judaism is with the publication and popularization of the Zohar (and the teachings of the Arizal and his contemporaries as well).
True, it’s not necessary to believe in gilgulim to have the right motivation. It’s just one possible source of extra motivation.
I guess another practical benefit could be that in some circumstances, a great tzaddik counseling a person with a lot of problems might be able to help them by figuring out that the problems derive somehow from previous gilgulim. (Perhaps like past-life regression therapy.) The Arizal did something similar, I believe (he was known for telling people who they were a gilgul of, and such things).
These are really just some random thoughts — I’m not a chosid or kabbalist or an expert on this topic by any means.