EFVs have no weaponry and weak armor, but you can manually switch them on to have either of three very special abilities:
Electronic counter measures – all nearby allied units (except other EFVs) will gain stealth capacity
Electromagnetic pulses – all nearby buildings (including own and allied) are powered down. This will often have ripple-effects; if a TFT FOC is powered down, all structures powered by that FOC are also shut down.
Improved targeting – increases all nearby allied unit’s damage by x1.25. Effects are cumulative if several EFVs have this power switched on.