When you go on a trip, do you just throw everything into your suitcase? Your toothbrush and toothpaste in the same compartment with your swimsuit, running shoes, dress shirts? If you do, I'm sure you've had a few surprises after the gorillas have thrown your bags around a bit. The organized traveler uses several small bags and pouches to keep belongings intact and looking their best. The same holds true in After Effects.
If your timeline has more than 20 layers, you should really consider organizing it better. Precomposing allows you to take several layers and put them into a new composition. They still live in your timeline, but are far easier to manage and track. To Pre-Compose, simply highlight the desired layers and press Shift+Cmd+C (Shift+Ctrl+C).