Why would something move against its concentration gradient

The only way in which something can move against its concentration gradient is if energy is involved. If an organism uses energy to gain something (normally via active uptake), it's because it needs a high concentration of it, one which isn't found in its natural environment.

It would move against it's concentration gradient when that thing is moving using active transport because it is a low concentration moving to a high concentration. The bigger the gap between the concentrations the steeper the line on the concentration gradient.

Hope this helps!

Think about a plant. It needs water. Eventually the plant will have a higher water content than the soil, but it still needs more water. Hence needing to move against the concentration gradient.

There can be other potentials that can compensate for the chemical potentials such as concentration gradients, e.g. an electric potential.