Unimaginable Power:
If equivalent masses of matter and antimatter are brought altogether, then in theory all the mass will be transformed to energy. If there happens to be more matter than antimatter, there will be few matters left over after the encounter. In opposition, if there is more antimatter than matter, there will be little antimatter remaining.
In a nuclear reaction, only a minute fraction of the mass of the constituents is liberated as energy; abundance of matter is always left over, though its form has changed. You may push altogether two chunks of 235U, the isotope of uranium whose atomic mass is 235 amu, and if their combined mass is great sufficient, an atomic explosion will occur. Though, there will still be a considerable quantity of matter remaining.
We may say that the matter-to-energy conversion effectiveness of an atomic explosion is low. In a matter-antimatter respond, when the masses of the samples are equivalent, the conversion efficiency is 100 %. As you can imagine, a matter-antimatter bomb would make a conventional nuclear weapon of the same total mass appear like a firecracker by comparison. The single matter-antimatter weapon of modest size could simply wipe out all life on Earth.