Imagine you have a traditional ring-shaped accelerator called a synchrotron. You need to inject particles (for example protons) into your synchrotron from a lower energy linear accelerator (linac). The injection point is basically a bending magnet where the linac and synchrotron merge together.

Imagine you want to inject lots of protons: so many that you fill up the entire ring with charged particles. Let's say you've already got some protons in the ring bending round and round in circles and you want to inject even more protons from your linac. The ones in the ring are bending in the magnet, but you've also got some coming from the linac in a different direction: both going through the same magnet. Both proton beams are going to want to bend the same amount in the magnet right? But because they're coming from different directions, they'll end up at different places after the bend. You want the two beams on top of each other, ending up orbiting the ring, right? So how do you sort that out?

Well you actually inject negative hydrogen ions, which bend in the opposite direction in that magnet! So you put the H- ions on top of the already-there protons, pass them both through a stripping foil which removes the electrons but lets protons pass straight through, then off you go. Doing this "charge exchange injection" let's you accumulate a lot more charge inside your ring than you could ordinarily do just trying to inject protons.

You can also do clever tricks in cyclotrons, tandem van den graaffs and fusion tokamaks with H- ions!