I've found, over the years, in situations like these from time to time, and only recently have I begun to understand why. You see, working spells to ensure that something happens can sometimes be akin to opening a door. Once that door is opened, you have to deal with whatever that door was separating from your presence. Sometimes it's pleasant, and sometimes it is miserable, frightening, and damaging.
When you open the door and find yourself needing to placate what you've found, tributes become important. Making offerings becomes a method of survival - as with life, you buy your allies.
This idea is suggested to in stories like what happens with the dead when they try to cross the river Styx...they must pay Charon for safe passage, lest they simply be left on the river banks.Just something to consider.