Grease is used to slow the wearing of the threads and keep o-rings suptle. The o-rings are part of the waterproofing - a dry or broken o-ring will allow water access to the inner workings of the flashlight.
Body extensions (aka run time kits) are used to expand the battery compartment space so the flashlight can be run on more batteries. This also makes the flashlight bigger for a larger grip area.
Battery adapters are parts inserted into the battery compartment to allow a flashlight to run on a smaller battery. These can range from a sleeve to keep thinner batteries stable to a post that completes the connection for a shorter battery.