Concept:
E-glass is an electrical-grade glass with good electrical insulation and mechanical properties.
Step 1: E-glass is commonly made into glass fibers.
Step 2: These fibers are used to reinforce polymer materials.
Step 3: E-glass fibers are widely used in:
• fiberglass reinforced plastics,
• electrical insulation,
• printed circuit boards.
Step 4: It is not mainly used for bottles, windows, or lead crystal decorative articles.
Therefore, E-glass is mainly used as:
\[
\boxed{\text{Reinforcement fibers for plastics \& printed circuit boards}}
\]