People who like understanding the bigger picture like knowing where and why major plot points come into being.

Example: Arachnophobia showed us where the giant, deadly spiders came from. There was a clear line of causality from jungle to mating with a house spider (easy enough to suspend disbelief) to infesting the town.

If you're introducing a biological monster, then having a backstory, an origin, if you will, adds the necessary layers of credulity for any reasonably critical viewer. Otherwise it may as well be the hand of god coming down from the clouds and making a tree, a deer, a bush, a xenomorph, etc.