What is the main advantage of scramjet and ramjet engines?

The main advantage of scramjet and ramjet engines are, it allows the vehicle to achieve greater speeds with higher efficiency and they do not have rotating or moving parts.You can read about the Hypersonic Technology Demonstrator Vehicle (HSTDV) – unmanned scramjet technology of DRDO in the given link.

Further readings:

1. Advanced Technology Vehicle – India, one of the 4 Countries To Develop Scramjet Engine
2. Reusable Launch Vehicle – Technology Demonstrator (RLV – TD)