Engineering mode expresses numbers in the form $a\cdot 10^n$ where $n$ is a multiple of $3$. This is well suited to mentally convert into SI unit prefixes (kilo, mega, giga, tera, etc. and milli, micro, nano, pico, etc.). If you do not have such conversions in mind, scientific notation is the nicest way to have an immediately visible size / order of magnitude. For numbers between $0.001$ and $9999$ I would certainly prefer FLO mode though.