Ingame IG have perhaps the best synergy with vehicles of any facton in the game. Just being ordinary men they are very squishy and have a bad tendancy to die when hit by anything sharp. Now while i wouldn't recomend charging mass amounts of Guardsmen into combat with space marines, i 110% recomend keeping your enemy at range and overwhelming him/her with mass amounts of firepower. The Imperial Guard very much adopts the strategy of overwhelming massed firepower at range and following up with massive infantry or armoured charges to rout the enemy, and this is very similar to how they play in DOW.

Infact the amount of Guardsmen in the Emperor's service at any one time is simply unknown, there is too many to keep track of. Just ordinary men, massed into huge regiments of men and equiped with only basic weapons and armour they are told to hold the line or die trying. In the background of the 40k universe the Imperial Guard are the main force that keep the Imperium safe from aliens, heretics and pretty much anybody who holds a grude with the Emperor.