We all use presets. They are very powerful and handy to use, but sometimes, when there are many expression controls involved it starts to get very messy. And that's where CustomEffects come in.
Even though CustomEffects are standard presets they look and behave as standard plugins. Adobe After Effects need to know the definition of the preset in order to interpret them correctly.
Some time ago started to publish my heavily expression based presets as CustomEffects.