The behavior pack makes tree leaf's fall off trees once in a while and allows you to do some commands such as 

/function start       (working on this)

/function desert    (adds sand particles and makes sandstorm-beta)

but im still working on these, you can still try them if you want to

furthermore it also enhances graphics depending on where you are (eg-underground) changes fog for a better view of ores and dark places

This also allows clouds to move around which looks awsome

This pack is needed as it is the main pack and the behavior is just an extra

Adds realistic Cube map and changes fog, light, brightness, shadow, water and water fog in all biomes

changes how leaves look, improves water lily, potatos, wheat and makes grass flow

and changes nether/end portal colours

glowy sun and moon phases

better flipbook textures, eg- realistic fire

better terrain textures

Makes cool sand, snow, leaf and cloud particles