Infantry units with the Leader capability are not meant to participate in the fighting! The player should field as many as he deems necassary and place them where they are needed, preferably put on return-fire. Any unit within a leader's area of effect
receive +1 experience level
the by far most prominent advantage of an additional veterancy level is the +1 accuracy
incoming suppression damage is halved
generally units get pinned much earlier than they get destroyed; pinned units stop shooting, and are only susceptible to the retreat order
particularly PaK 38 50mm in heavy cover, S.MG 42 and other long-range infantry in heavy cover
do not surrender
it is not easy to create the situation to make one or even multiple units surrender, as retreating units run particularly fast, but it happens, e.g.
when pinned units do not recieve the retreat order in time
during close-quarter combat, when enemy units are already very close
units next to a road that were surprised by bombing run, can be made to surrender by a high speed vehicle
are not affected by morale penalties behind enemy lines
units behind enemy lines do not recover morale! Should they get pinned, they remain pinned, should they receive the retreat order, they will keep retreating as long as they remain behind enemy lines. Units with the airborne capability are not affected by this neither.