if we build machines for the purpose of antimatter production, we might be able to use it to propell a small unmanned space probe on long distance (beyond jupiter or saturn where the sun is quite far away already). Just like using fuel cells in the Apollo programs, it's incredibly wasteful of $$$ as an energy medium, but for space use it might be considered, as launching heavy-weight stuff is even more wasteful.
An anti-proton beam might also be used to destroy cancer cells, I remember research in that direction a few years ago ; the beam can be precisely collimated so it toast just at the right distance, whereas old-style cold war radiations damage cells on the whole trajectory.
And as a weapon, it feels stupid. Use nukes instead, you can afford lots of them if you're able to build a antimatter weapon anyway. And I don't know what's the use of a hand grenade with the power of a Hiroshima bomb.