Since I've always been someone who has been pretty much aware that we don't have the people we love forever, I can't say I learned more than I already knew about how we need to cherish our time with those people while we have it. Instead, I have to say that losing loves ones has made me more and more skilled at coping with that kind of loss (but also smaller losses), as I've perfected "techniques". It's helped me know what works best for me, but some of those losses have made me particularly aware of how little time any of us (even those who live to be 100) have, how quickly it passes, and how there really isn't a lot of time to waste on what doesn't matter to us.
I guess, too, having had those losses has meant I truly understand the shorter term sense of horror (among other things) that can come with it, and the "lower level" kind of sadness that can diminish in "sharpness" over time but that generally can always "stay there" somewhere. Before we've had one of those losses, we know it's an extremely awful thing to lose someone. Once we've had one, or more, of them we have a better idea of the many mental and emotional dimensions and facets there can be. With enough of those losses, we also learn how, while they can be similar in some ways, each one has its own unique set of circumstances and "issues".
Finally, I've learned how to live with a "collection" of those losses and still keep finding my way back to the same, old, happy enough person that I've always been (at least for the most part). I've learned, too, that even though it takes a long time, we do eventually find our way back to the person we've always been (but that it can take longer than those who haven't been through such a loss, and even we, think it should take).