Why does pre-rendered footage get re-rendered again for every playback or preview?

Dear Sir/Mdm,

I'm a novice, please bear with me.

I'm using an Asus laptop with:

     1. CPU: Intel i5-3337U @ 1.80 GHz (4 CPUs)

     2. OS: Windows 8.1 64 bits (6.3, Build 9600)

     3. RAM: 8 GB

     4. DirectX: Version 11

     5. On-board Graphics: Intel HD 4000

     6. Dedicated Graphics:  GeForce GT 740M

     7. Dedicated video memory: 2048 MB DDR3

     8. Nvidia Driver version: 6.14.13.3523

     9. Total available graphics memory: 4096 MB

I hoped to avoid repeated rendering of footage segments during RAM previews, scrub-through, or playbacks.

I pre-composed related layers of completed segments and pre-rendered the pre-comp, which gave a footage (in AVI format) to replace the pre-comp.

But when I scrub-through, playback (spacebar), or do RAM previews, the pre-rendered footage gets re-rendered again (for every scrub-through/preview/playback).

Need advice.

Thanks.