They help you deal with shade from trees and other buildings so that your system performs at optimal levels.
Optimisers, as the name implies, maximise the energy output from solar energy systems by constantly tracking the maximum power point of each individual module.
You need optimisers to deal with shading issues. Traditional solar PV strings without optimisers are entirely affected by shading on a single panel. Thus, if shading affects a single panel by 50%, this brings down the efficiency of a single string by 50%. Optimisers allow solar panels to work independently in the string so that only the output of the panel concerned is affected. Thus, if one panel is not performing at optimal levels, the rest can still work at maximum output.