Most people do not want their units to die. Some of them have quite sound reasons:
1. When one on one firing (ex: 6 aa vs 6 aa), retreat the unit that is being trageted can make your opponenet's units chaise that units most of the time (can't chaise and fire in the same time and you can shoot moving units if they are in your traget)
2. Their units have other responsibility (ex: gg peg looking for nukers, no time to do one on one fight)
Others unfortunately have less sound reasons:
1. still believe the myth that alive units in a win battle gets 100%xp, alive units in a lost battle gets 75%xp, dead units in a win battle gets 50%xp, dead units in a lost battle gets 25%xp (believe me, many people still tell me this). This is clearly not true since the max. xp in a bttle for one single unit is 95% and the amount of xp you get for a units is not base on the factor above only, it also depends on your unit's level, time in battle, number of kills, level of units you killed, relative army strength).
2. Love their units too much, just can't bare to see their units to die. (I call these people units hugger)
I will not deal with the bad consequence of being a "units hugger" in this post, but I will talk about the advantages for sacrifing your units AT THE RIGHT TIME.
A. When you notice there is a mine field ahead(air or ground)(you know it because you already ate some of the mines), many "new player" or "units hugger" will just fly/run back without noticing other people in the chart.
What you should do is 1. run the mine field using formation Q (doesn't matter when and what units you are using*, since most mine are over important areas (ex: near poc or portal, path to poc or portal) there are no way to avoid them and go to where you want in the same time unless you are very lucky) 2. notice people in the chart (yes, most people do not read the chart, but you do your job), you can either ask people to clear it for you (*you are using end-gamepig or nix, don't run away with you lvl 20 pelican, etc.) or notice them there is a mine field AND you clear it, THEN tell people if mines are cleared or not.
B. When the poc is almost finish poccing/repoccing in end game time
Get your units on poc!! Don't care if the poc is being arbed, pigged, mobbed (of cause do it with skill such as press "S", unclock units one by ome if possible while carefully moniting the situation, move within the poc to avoid pig). There are times whcih I have 42 pegs to crazy level my mob level (and yes I use ga pegs to end game), I always put my pegs on poc when there are no other units willing to do so or fast enough to get on poc and i can say 80% of the time that poc is pocced, it DOES matter what units you use to poc, but IT MATTER MORE if the units ARE ON THE POC, no pint to use 24K hp morthlas fragging around the poc when it is end game time and YOUR FACTION need that poc to WIN !!
C. When killing enemy arbs, balls, trebs, nukers, pigs, nixs
if you kill those units fast, it helps other A LOT, stop running away, there are not many time that the units mentioned above are unguarded.