Steve,
As mentioned above, this gives the fluid time to properly make its way down into the wicking. Many Lionel engines use an interior funnel to the unit because the exhaust wouldn't fit directly under the stack. This gives each drop a somewhat circuitous route to get into the unit, and that takes time. This is especially true on diesels.
The other thing I've read is that dropping cold fluid onto a hot coil causes the fluid to crystalize onto the heating element. I haven't done the science on this myself, but I've been happy with my results.
Jon 
