An unmanned probe can take an extra month or year to get there with no changes to the actual mission as long as the path is fuel efficient.
For human cargo, the time difference is as important as the fuel expenditure - extra life support supplies can be heavier than extra fuel to make the same flight faster, or the need for much longer life support may make the whole thing impossibly heavy.
For human cargo, the time difference is as important as the fuel expenditure - extra life support supplies can be heavier than extra fuel to make the same flight faster, or the need for much longer life support may make the whole thing impossibly heavy.